(

Cesium Heatpipe Experiment

Resonance Ionization Spectroscopy of Cesium Absorption Spectroscopy of Cesium

Stimulated Electronic Raman Scattering of Cesium Fabry-Perot Interferometry with a Nitrogen Pumped Tunable Dye Laser

Reference Materials

.Stimulated Electronic Raman Scattering Grotrian Diagram for Cesium

."Stimulated Electronic Raman Scattering in Cs Vapour: A Simple Tunable Laser System for the 2.7 to 3.5 Ilm Region," by D. Cotter and D. Hanna, Optical and Quantum Electronics 9 (1977) pp. 509-518.

Required Equipment

.PTI Nitrogen Pumped Tunable Dye Laser System

.Cesium Heatpipe

.EG&G Judson IR Detector

.Photodetector PreAmp

.ORTEC Model 570 Amplifier

.Sample and Hold Circuit

.Data Acquisition System--DAS-20 ADinA Board and Software

.Optics--Prisms, Mirrors, and Lens

.Optical Hardware

C:\WINWORD\DOC\MODPHY01.DOC/09/12195

I

I

I

I

I Resonance Ionization I

,and Absorption Spectrum I Lab

I

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-1-------- ._ .. - _ I

Introduction

It is possible to accurately and precisely map the excited energy shells of a cesium atom by constructing an experimental apparatus consisting primarily of a tunable dye laser, an optical "heat pipe", and an optical detector. This setup allows an assortment of possible measurements to be made, each providing unique and valuable information about the atomic processes and structures of the atom. In addition, the apparatus only has to be varied slightly between each different measurement, therefore resulting in a versatile, accurate, and quick means of data acquisition. Using this apparatus, we can produce resonance ionization and absorption spectrums which allow us to calculate the excited energy bands of a cesium atom.

Theory

I

The Cesium atom, atomic number 55, is a metal with a ground state electron configuration of[Xe]6s1. In this ground state, all electrons remain in a bound state, with the electron in the 6s level being the farthest from the nucleus. Experiments have been performed which indicate that it is possible to strip this electron away from the atom, (i.e. ionize the atom), if an ionization potential of 3.893 volts is applied to the atom. In addition, quantum mechanics predicts that there will be a number of discrete energy levels

I

between the ground state and the ionization level or free electron state. It is these discrete excited energy levels which we would like to measure. According to quantum mechanics, it is expected that the energy level diagram looks something like figure #1.

////////////// J ----- 9S 1/ 2 7d ----- 8P'/2. )/2 ----- '/2,)/2

----- 8s 1/ 2I 6d 1/2. )/2

I ----- 7S'/2 =====5d5d'/2 1/2

I I I According to the selection rules of quantum mechanics, an electron going from the ground

state 62S1/2 to an excited state, is prohibited from jumping directly to the 62P, 72S, and, 52D states, therefore, leaving only the 72p state as the lowest possible state to which an excited electron can jump. One of the ways by which an atom can gain the energy ) necessary to push an outer electron into an excited state is by the absorption of a photon.

--I

Figure #]

I

I

I

This photon, with energy E = he, can only be absorbed by a cesium atom in the ground }.

state if the photon has exactly the value of the energy difference between the 7p and the 6s I states. Therefore, if a photon of the right wavelength, and consequently the right energy,

encounters a cesium atom, it will be absorbed causing the atom to become excited. In addition, if the cesium atom, while in this excited state, encounters another photon of theI same wavelength, this photon will also be absorbed, resulting in enough energy to ionize the atom. If this two-photon ionization process is performed on a large number of atoms simultaneously, then the numerous electrons and positive ions which result will, if a potential difference is applied, create a measureable current. Therefore, when our photon energy corresponds to the energy gap between the 6s and 7p state we will get a large current, while when our photon energy does not correspond to the 6s and 7p energy gap we will measure a near zero current. Consequently, by using a tunable dye laser to scan over a range of wavelengths while recording any changes in current value within a cesium

I gas, we can produce accurate measurements of the energy gaps possible betweeen the 6s and 7p states of cesium. The graph that results from these measurements is known as a resonance ionization spectrum and it only measures those atoms which have been ionized

I by the two-photon absorption process. Inevitably though, there will be atoms inside a cesium gas which will not be ionized. This is due to the statistical fact that some atoms will encounter only one photon, therefore, become excited but not ionized. Consequently,

I no electron is produced. However, it is possible to measure this photon absorption by detecting the amount of light which is transmitted through the cesium vapor. When the wavelength of the light approaches the expected energy gap between the 6s and 7p states,

I more atoms will begin to absorb the incoming photons. Therefore, a drop in the transmitted intensity can be easily observed and measured. The graph produced from this type of measurement is referred to as an absorption spectrum and it, once again, reveals I information about the structure of the excited energy states.

I Procedure Our experiments consisted mainly of two set-ups. The first of these was the

I construction of an apparatus to measure the amount of ionization occuring within our optical heat pipe. This set-up is illustrated in figure #2.

I

I

,..."J

•

I

Computer ~ A/D converter

rh ~ ~~ig. ,....-----fOutput

Amplifier

input

Ch.2

I Ch. 1 #2

Out

trig.

out

#2

In

Oscilliscope Sample and Hold

trigger in

delay/gate generator

Ext. trig. in

output

Function generator

trig. in A

he.at pipe

I Bout

C output

output

from heat

pipe charge

collector

M ~

N 2

laser

IE' .'-__--..J xt. tng. In

.1

I

I

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I Figure #2

I The second set-up that we constructed was designed to provide information about the

absorption of the cesium vapor. This is illustrated in Figure #3 . (note the introduction of a visible wavelength photo diode) .

I

I

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I

'J -1I

I (

I- A/D converter AmplifierComputer

I .---- ofJutput

input

Ext. trig. in

heat pipe

#2trig.#2

Inout

C output

I

trigger in

Out

Sample and Hold

trig. in A Bout

delay/gate generator

rh "" ~~ig.

I Ch.l

Oscilliscope

output

Function generator

Ch. 2

I

I I I I I:l

~ I

laser

I '--__.....II Ext. trig. in ..J

I Figure #3 Once all connections have been made we are ready to begin aquiring data. To load the

I necessary software type CD QB45 at the c: prompt. Tken type QBfL UT.QLB to start the quick basic program and its necessary quick library. Use the mouse to load the file named UTPRADAQ.BAS. Orice the file has been loaded, use the mouse to start the

I program. The program is relatively self-explanatory and should be mastered rather quickly. Use this program to perform both the resonance ionization and absorption spectrum experiment and make sure to get a hard copy of all results. The expected

I excitation wavelengths according to the National Bureau of Stardards are 455.5276 nm and 459.3169 nm.

I I I

··1---1

--- - -

--

---

'~ ;~}Ij',\;~f

+:lu 1'- V

'= "'"

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"'

PRA LASER SCHEMATIC Drawn: J, E. Parks April It 1993

----'---J } •. . .~,. ~~~\".

,... 241000~1 (S)t 1.00~~r---(S)o· Ln ........-I

,,< ~ '_._0 . ,1""1 ~t-2.738 ~~.A

-

Ij~_... ~

unnn ••••

SHV Connector VCR Assembly Female Nut (Rotatable) The flats on the conflat flangesand Gland on each end are not necessar~

OPTICAL HEA TPIPE Drawn: J. E. Parks MahJ 15)

Scale: 1" "" 5"

1993

.C:ET A'\.(1) 1\ ~....

TEMPERATUi\Z' DEGREES CENTIGRADE - .....-.... M

, ~· '.".l I'

'.f '.

" ".. .' I

"

.\,. \

~ .~, .,. ~ 'l

10. 2

1500 2000 3000 4000 .SOOO 6000 8000500 600 700 aoo 900 1000300 400 ~ .- ,.... .. -. .................TF.MPERAT\JRE DEGREES KELVIN~ MELTING POINT~ tlPT'l"alll .tll

------- ESTIMATEO

10.12

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;;.

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D~e Laser Scanner

Aboorl>flon C

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--- --- ---- f---- - - -~-,--UJ L UJ

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PUT

-15 V ... j5 V3 I

TRI ~ER 1 16

2 15

.01 uF 2

oun UT 3 14

'1 13 + 7I Outpu5 74LS22112 :I 6 11 IntExt Ref

CampHold Grd7 10 -V 5I k :5 11 6 8 :8 9 :0.1 C

__I f _: H -15V _. - --------If -------

C H

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---J Vee - 5 V

TRIGGER IN

~~ ::I i r-. 3 14

4 13 i I .5 74LS221 12

l: 11t--t--t 10~ TRIGGER OUT

r---..2.2 uF ~ TO AID CONVERTER

TRIGGER IN 1 k

9

-=

SAMPLE & HOLD CIRCUIT Drawn: J, E, Parks June 10, 1993

..Vee - 5 V • S&HSCHEM,FCO

IN

2.2 uF

• •

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:, ..!.~ ! ~.,..;,;.;;.

\

GROTRIAN DIAGRAM FOR CESIUM

Blue 7

2 01/2,3/2+.9 25 1/ 2

6 4OV/ 2,3/2 825 1/ 2

, (J .,

72 p1/2,3/2

~

7 25 1/ 2 /

5201/2.3/2

62 p1/2,3/2

'77777M"77.//II»~»~»~»7>7>,,,,6 251/2 Jj i L j) >In>

UNITED STATES DEPARTMENT OF COMMERCE. Maurice IL Stans" Secretary NATIONAL BUREAU OF STANDARDS I..ewU H. Branscomb Dil'ecwr

ATOMIC ENERGY LEVELS AB Derived From the Analyses of Optical Spectra

Volume ill

The Spectra of Molybden~ Techneti~ Ruthenium, Rhodium, Palladi~ Silver, Cadmium, Indium, Tin. Antimony, Tellurium, lodin~ Xenon, Cesium, Bari~ Lanthanum-Hafnium, Tantalum, Tungsten, Rheni~ Osmi~ lridi~ Platin~ Gold, Mercury,

," Thallium, Lead, Bisinu~ Polonium, Radon, Radi~ and Actinium if!

BY CHARWTTE E. MOORE

NSRDS-NBS 35

Nat. Stand. Ref. Data Ser., Nat. Bur. Stand. (U.s.), 35fV.m 282 pages (Dec. 1971) CODEN: NSRDA

Reis8ued December 1971

Reprint of NBS Circular 467, Volume Ill. See author's note opposite title page

____ £QLsJ!le~y the Su~rinlendentof Documents, U.S. Gov~rnment Printing Office, Washington, D.C. 204.02 . (Order by SO Catalog No. C13.48:35/V.lll), Price SS.4S domestlCPoslp8}

124

CESIUM

Cal

55 electrons Z=55

68~ 31406.71 K 1. P. 3.893 volts

A number of references have been used to derive the best available level values from the separate. sets of wavelength measurements, starting with the ground state zero. For the principal·series and the limit quoted above, the results of Kratz are quoted. He has observed this series in absorption to n=73. McNally and his associates have also carried the observations of this series in absorption to n=62, and give the limit as 31406.32 K. For the ID series Ma.ek has furnished his unpublished observations of the forbidden lines 68 JS-nd !])(n=9 to 21), for inclusion here. The work of Meissner and Weinmann has provided data. for some 'D, JS terms and for the 'F0 series. Meggers' observations furnish revised values for 6p TO and &PD.

The well-known series from Fowler and Paschen-Got.ze are given by Bacher and Goudsmit. Their terms that still need to be re-evaluated from more accurate observations have been rounded off in the table, namely, 11, 128 JS; 5, 6g 20; aDd 6h IBo. New measurements in the near infrared furnished by Fisher in advance of publication, give the connection of the 5d 'D and nf'Fo terms with the rest of the terms, and the quoted value of 7825.

Beutler and Guggenheimer have classified 53 ultraviolet lincsobserved in absorption, between 653.60 A and 1007.5 A, as transitions from the ground term to levels above the ionization limit. The high limits are from the Cs II analysis. These autho~ have classified four groups of lines called A, B, C, D, having the respective limits in Os II .p~,llp~, llP:, and l~. The value of the limit they use for group C (3P:), 150870K, has since been replaced in Os II by one at 153772 K. The writer has, therefore, attempted to adjust the series for group C by assuming that their first series members 5d and 78 are correct as given, and adopting the best available lines that fit the revised limit. Consequently, their level at 142577 K is now labeled 6d? instead of 88; their level at 139694 K, formerly called 6d, has been rejected i and the 88 level is tentatively placed at 144501 K. The miscellaneous levels, which are the wave numbers of the observed ultraviolet lines, have been assigned numbers by the writer. The double entries of J for these levels (unlike those for unresolved terms) indicate that the

. . .... existing data are insufficient to determine whicli value of J is correct. The ground state 68 280u has ahyperfIDe structure with a separation of about 0.30 K

observed by Kratz from 7 to 72p 2PO. Mack has also observed it in the 68 28-00 2D observations from 11.=9 to 15.

More than 200 lines are classified between 653.60 A and 7.4 Il.

Cs l--eonunued

REFERENCES

Contig. Desig. J Level Interval qonfig. Dcsig. J Level Interval

51"(18)61 &'8 O~ 0.00 511'(lS)10., 10. 2S O~ 28300. 28

&pI('B)6p 6p lpo O~ 1~

111'l8.~.e

11'l3f.lJ6 554. 11 611'(lS)61 61 IF" 3~

2~ U3t9.880 u:!t9.781 "";0. 102

~ 8)5d)'~ saiD 1~ 2~

14499.490 14597.08 97.59

511'(IS)6g 6g '0 a~4~ 28347

511'(IS)6h 6h 'Ho 4~5~ UtJ56 51J'{1S)7. 7, 'S O~ 18535. 51

Sp&(18)7p 7p 'PO O~ 1~

~1'l65. 65 1!19.e6.66 ISLOI

511'(18) lOp lOp 'PO O~ 1~

187t1.09 18753.93 26.84

5;'(I8)6d M'D 1~ 2~

22.'>88. 89 2263L 83 42.94

511'(1S)9d 9d 'D 1~ '2~

28828. 90 28836.06 7.16

511'(1S)lh 111 '8 O~ 29130 5p&(1S)& 8s 'S O~ 24317.17

~4f 4!'F" 3~ 2~

~«1t.~ f44'lf.483 -0.176

511'(!S)71 71 'F" -

3~ 2~

lDl+B.156 lDl+B-IM -0.06

s,'(IS)8p 8p 'PO O~ 1~

tS709.14 tS'l91.78 82.64

511'(!8)l1p Up 'PO O~ 1~

lD40$.68 lD#1.10 17.42

Sp&(lS)7d 7d 'D 1~ 2~

26047.86 26068. 83 20. 97

611'(1S) 10

126

Cs r-eontmned Cs r-eontillned

CoDfig. Desig. J Level Interval Config. ,

Desig. J Level Interval

6pI(I8)13p 13p apo O~ 1H

tJ()168.00 8017+61 8. 51

6p'(1Sj:Up Up apo 0~,1~ ~l1U 1~

S"(I8)l2d 1U aD 1~ 2H

30197.02 30199.35 2.33

6p'(1S)25p

51"(1S)26p

25p apo

26p apo

0~,1~

O~,l~

31168.0~

~1188.87

5"(18)10/ 10/ aF" 3~ 2~

tJ()SOf. ~8~ ~OSO:.890

-0.007 51"(1S)27p 27p IpO O~,l~ 3leO? 07

6pI(tS)14p 14p ,po O~ 1H

80393. 16 tJ()899.~

6. 33 51"(IS)28p

51"(IS)29p

28p ,po

29p apo

O~, 1~

O~, 1~

~1!f3.09

81!87. t~ 5p'(1S)l3d 1ad'D 1!i

2~ 30416. 06 30417.76 L 70

51"(1S)30p 30p IpO O~, 1~ 81t~.81

SpI(tS)l1/ 11/ 'FO 3~,2~ 8049+ 59 51"(1S)31p 31p 'PO O~, 1~ ~1t~J1. 05

S1"(1S)15p 15p 'PO O~ , 1~

30563. t7 80567.98 4.71

51"(18)32p

51"(1S)33p

32p 'po

33p ,po

O~, 1~

O~, 1~

81t71.18

81taO. 17 51"(lS) 14« 144 'D 1~

2~ 30580.46 30581. 79 1. 33

51"(1S)34p 34p ,po Ox, 1~ 81ta8.36

51"(18)12/ 12/ 'FO 3~,2~ :J061t0.9S 51"(18)35p 35p ,po 0~,1~ 81!95. 76

6p'(lS)16p 16p 'po O~ 1~

30693. 76 80697.5t 3.76

51"(1S)36p

5pI(1S)37p

36p ,po

37p IpO

O~, 1~

0~,1~

3130B.51

81808.61 5p'(lS)lSd ISd ID 1~

2~ 30707. 11 30707.96 0.85

51"(IS)38p 38p 'PO O~, 1~ 3181+ Bl

51"(tS)17p 17p 'PO O~ 1~

30798. to :J0799. 15 2. 95

51"(1S)39p

51"(IS)40p

39p 'po

40p apo

0~,1~

O~,l~

81~19.~9

818t+ 11 6p'(lS)16d 16d 'D 1~

2~ 30806. 59 30807.2 0.6

51"(1SHlp 41p 'po 0*,1* 818t&. ~7

ti1"(1S) 18p 18p 'PO O~ l~

80878.07 80880. +.1 2. 34

S1"(IS)42p

51"(18)43p

42p 'po

43p ,po

O~, 1~

0~,1~

~lS8t. ~7

81~36.19 6p'(1S)17d 17d ID 1~2~ 30886. 7

51"(lS)19p 19p 'PO O~ 1~

809-U. 49 80946.~

1.94

51"(1S)«p

51"(1S)45p

«p 'PO

45p 'po

0~,1~

O~,l~

81389.67

818+£.84 61"(I8)lSd lSd'D 1~,2~ 3095L 5 51"(1S)46p 46p 'po O~,l~ 81845.80 5p'(1S)20p 20p 'po O~

1~ tJ()999.15 81000.7. 1. 59 51"(18)47p 47p ,po O~,l~ 81848·68

61"(lS)19d lSla'D 1~2~ 31005. 0 S1"(1S)48p 48p ,po O~,l~ 81851.18

6p'(1S)21p 21p lpo O~ 1~

, 810++ 68 310-'8.00 La7

51"(1S)49p 49p 'po O~,l~ 81858.67

61"(lS)2Od 20d 'D IX, 2~ 31049. S 51"(1S)OOp OOp 'po 0~,1~ 81856.86

51"(tS)22p 22p ,po Ox, 1~ 8108+. 08· 51"(IS)Sip 51p 'PO Ox, 1~ 81857.98

5p'(1S)21d 21d 'D 1~,2~ 31086. 7 S1"(IS)S2p 52p 'PO 0~,1~ 81859.95

5p'(IS)23p 23p 'PO 0~,1~ 31118·40 51"(lS)53p 53p 'po O~, 1~ 31361.80

-------------- ------------ -------- --------------------

--------

Cs l--eontinued Cs l-Co tin

J Level Interval Config. Desig.

"p tpo Jpo~I:183. 600~1~ 1.6" 6I~Pl)1.6" & PJ)1.65p tpo ~1$86.t30~1~ 6" 61('PJ)5d 5d 10

SGp tpo ~1:JQ6.18O~I~ 4po6" 6I('pt)1. 1.

r;Tp tpo . ~1:JQ8. ~1O~I~ 6" 61('PJ)6d 6d 8°

SSp tpo ~J~9.690~1~ 5" 6c('PJ)6d 6d 9°

69p tpo ~1370. 96O*, 1~ 5" 6I('PI)6d 6d 10°

60p tpo ~131LMO~I~ 6" 6c('PJ)6d 6d 11°

61p tpo 31313.#O~, 1~ 5" 6I('P"i)6d 6d 12°

62p tpo 3131+ 560*, 1~ 4po5" 6I('P;)5d l>d'

63p Jpo 3J815.610~1~ Jpo5" 6c(lpt)5d 5d'

64p tpo ~1376.610~1.~ Jpo5" &(,P"J& 86

651' Jpo 31371.590*.1~ 6d 13°5" 6I('P"J6d

O){,l~ 31378.56 ~po61" &('pt)& &

0){,1~ 31819.46 Spi &('Pi)1d 1d 14°

31580.£90*, 1~ 1d ISO511' &('Pi)1d

31581.080*. 1~ , Spl &(JPI)7d 7d 16° 31581.870*, 1~

g" 'po5" & ('PI)9&1 31381.670~1~

6" &('PJ)5d Sd' 110 31:J8S.f8O*,l~

5" &(lpoJSd Sd' ISO,,,r6 ~1:J83.65Oh,l* 51" 6s('PI)8d 8d 19°

9. 2005" &(,P"J9.sT

n«oo. 71 5p16a('pt)8d 8d 21° ~po99169 Spi 6a('Po)7. 1~ 1.'-9810O){ 109069 2POSps 6s('P".) lOs lOs

1~ 113-491 Spi 6s('Pi)12a? 12. 'po

O){ 5p16s('pt)lh1114560 11. 22°

spo51" 6c('PI) 13a 1&1115390* -511' &('pt) 1211 121 23°1111071~ 61" 6s(IPi)1. 1.' tpo

1185141~

O~ . l1teS5 Cs n(lPl) Limit

Config.

pt(I8)"p

6p'(I8)~

6p'(I8)SGp

~57p

~J"(I8)SSp

~(IS)69p

eop pl{l8)61p

(l8)62p

~p

Mp

(18)65p

S)66p

(18)611'

(18)681'

11'(18)691'

(18)1Op

pl(l8)11p

(l8)12p

P'(I8)73p

Os u(lSa)

(IP()&! ('pt)&!

&('PI)5d

6a(lPJ)Sd

6a(lPl)5d

61(1pt)5d

&(lPI)5d

6alI Pl)5d

~

Ii

&II' \

DesIg.

661' spo

671' ,po

68p spo

69p spo

701' tpO

71p tpo

12p tpo

731' tpo

££mit

spo64"

Sd 1°

5d 2°

5d 3°

5d 4°

5d 5°

Sd 6°

n ued

J,

1* 0*

1*

1*

OM.l~

OM. 1*

0){,1){

0*,1){

0*,1*

1*

1*

0*

O*, 1){

1*

O~ 1){

0){,1){

0){,1){

1){

1){

O){

}O){,I){

0){,1){

O){

1){1

1){ 0*,1){

1){

0){,1~

O){ 1*

Level

1~.pJ8

1~

1~~

lU()18

1~t08

IM~

le7~

le7868

1e7973

1:J08t9

IfJ05n

l~U11

131t58

18150.{

131818

13t560

188706

18S584

1:J8906

18~11

t8~614

IM014

13518~

IM3',

}1S6865

131t97

131541

1:J7794 1:J88~

1:J8199

Interval

-107

1009

128

CSI--eontinued Cs I-Continued

Config. Desig. J Level Interval Config. Deaig. I J Level Interval

Os n(lpt)

6pI 6I~PI)6d1 6pI & pt)6d

6pI &(1pt)6d

6pI 6I(lPl)6d

6pI 61(1P1)Sat

6pI &(1pt)Sa

~6I(lPl)7d

6pl6I(lpt)7d

&pI &(1pt)7d

Limit

&i' .po 6

NSRDS7NBS 68

u.s. DEPARTMENT OF COMMERCE/National Bureau of Standards

Wavelengths and Transition Probabilities for Atoms and Atomic Ions

Part I. Wavelengths Part II. Transition Probabilities

I:

!!i Ce 111 Celli Celli Ce IV i .

Intensity 1000 3000

Wavelength 2151.44 III 2166.88 III

Intensity 4000 4000

Wavelength 2748.90 III 2754.87 111

Intensity 1000 3000

Wavelength 5983.40 III 6002.63 III

Intensity Wavelength 20 , 1779.03 IV 35 ! 1914.75 IV

2000 2169.48 III 4000 2768.28 III 10000 6032.54 111 10 1937.21 IV 5000 2180.64 III 3000 2849.40 III 10000 6060.91 III Air 1000 2183.71 III 2000 2861.39 III 500 6061.79 III 100 2000.42 IV 2000 2203.15 IIi 4000 2907.05 III 500 6097.35 III 35 2003.11 IV 3000 2218.11 III 10000 2923.81 III SOO 6098.87 III 100 2009.94 IV 5000 2222.01 III 5000 2925.26 III SOO 6135.10 III 3 2433.50 IV 5000 2225.08 III 10000 2931.54 III 300 6287.79 III 5 2445.50 IV 3000 3000 5000 2000 2000 2000 2000 2000. 3000

2227.84 2228.05 2242.29 2249.25 2264.85 2268.20 2287.82 2298.70 2300.65

III III III III III III III III III

2000 5000

10000 50000 95000 20000 40000 20000 40000

2948.53 2973.72 3022.75 3031.58 3055.59 3056.56 3057.23 3057.58 3085.10

III III III III III III 111 1Il 1Il

SOO 300

1000 700 300 SOO 300 500 500

6308.16 6341.75 6944.94 7739.04 7758.27 7826.80 7948.64 7960.31 7991.01

III III III III III III III III III

100 300 150 200 100

Cev Ref. 261 - J.R.

Vacuum 365.66 V 399.36 V 404.21 V 482.96 V 552.13 V

,

4000 5000

10000 5000 2000. 5000

2302.09 2317.34 2318.64 2324.31 2337.66 2350.10

III 1Il III III III III

20000 30000 30000 20000 20000 20000

3106.98 3110.53 3121.56 3141.29 3143.96 3147.06

111 III III III III III

400 300 400 300 300 300

8030.80 8084.12 8177.33 8186.03 8222.16

,9.056.53

III III III 1Il 1Il III Refs.

CESIUM (Cs)

Z = 55

Cs land II 82,130,154,155,200,

j I\: !~I;t "

2000 2000

2362.54 2367.77

III III

20000 3000

3228.57 3234.20

III III

300 400

9079.58 9328.20

III III

201,223,263,266,267,300, 325,326, - K.L.A.

! 10000 2372.34 III 4000 3267.76 III 300 9367.03 III Vacuum

5000 2377.07 1Il 3000 3267.94 III 300 9567.37 III 25 591.044 II 5000 2377.48 III 20000 3353.29 1Il 400 10458.37 III 5 607.291 II

10000 2380.12 III 10000 3395.77 III 400 10494.42 III 4 612.786 II 3000 2382.28 III 4000 3398.91 III 300 10534.36 III 200 639.356 II 3000 2385.06 III 30000 3427.36 III 400 10684.46 III 10 657.112 II 5000 2395.04 1I1 40000 3443.63 III 15 12756.96 III 50 668:386 II 3000 2406.15 III 30000 3454.39 III 12 12&21.62 III 1500 718.138 II 4000 2408.08 III 40000 3459.39 III 80 15847.58 III 1500 808.761 II 2000 2410.26 11I 60000 3470.92 111 80 15956.79 III 1500 813.837 II

'..'. 4

5000 2000

2415.60 2417.01

III III

50000 60000

3497.81 3504.64

III III

12 87

1'5960.59 16128.75

III III

3500 4000

901.270 926.657

II II

2000 2423.02 1Il 500 3514.41 III 42 18579.82 III 130 1178.65 II 3000 2428.64 1Il 50000 3544.07 III 38 19141.29 III 80 1191.55 II 5000 2430.24 III 3000 3784.29 III 27 19377.15 III Air

10000 2431.45 III 800 3936.80 III 26 19466.14 III 25 2025.05 II 15000 2439.80 III 300 3957.10 III 20 19498.14 III 60 2035.15 II

3000 2441.55 III 500 4169.42 III 55 19524.18 III 80 2077.43 II 2000. 2444.78 III 300 4191.70 III 30 20685.63 III 80 2080.05 II

10000 10000

3000 5000 8000 3000

10000 10000

3000 2000

20000 3000 3000 4000 2000 2000

10000 2000 2000 2000 2000 3000 2000 3000

2454.32 2469.95 2471.66' 2477.25 2479.44 2479.51 2483.82 2497.50 2503.56 2504.43 2531.99 2539.27 2557.49 2577.67 2578.30 2584.71 2603.59 2607.96 2615.79 2649.38 2662.81 2719.30 2730.04 2743.71

III III III III III III III III III III III III III III III III III m 1II III III III III III

500 300 300 400 300 600 400 600 500

1000 1000 300 500 300 500 500

1000 500 300 500 500

2000 500 400

4194.83 4213.26 4217.13 4284.77 4304.71 4346.35 4389.97 4448.32 4485.27 4521.92 4535.73 4576.90 4627.60 4766.07 4976.45 5650.97 5664.20 5691.08 5710.59 5749.47 5949.83 5962.22 5962.71 5979.56

III III III III III III III III III III III III III III III III III III III III III III III III

12

2 1 8 8

40 30 12 6 5 2 9 1

50 75 75

2 1

15 20

21380.23 III

Ce IV Ref. 166 - J.R.

Vacuum 447.58 IV 443.11 IV 558.92 IV 571.59 IV 741.79 IV 754.60 IV 755.75 IV 975.20 IV

1009.31 IV 1022.12 IV 1057.67 IV 1059.64 IV 1289.41 IV 1332.16 IV 1372.72 IV 1577.60 IV 1572.62 IV 1641.58 IV 1775.30 IV

80 80 15 25

100 130 25

130 25

100 130 250 200

25 350 350

25 25 40

130 80

130 25

130

2088.71 2091.97 2099.50 2112.65 2146.75 2179.60 2182.14 2189.47 2213.15 2220.51 2228.88 2254.58 2257.82 2258.35 2267.61 2273.83 2286.68 2307.71 2315.68 2321.07 2343.13 2354.44 2357.85 2364.81

II II II II II II II II II II II II II II II II II II II II II II II II

i~ j;,(

il n '. ii I'

i' I' !

\

-) 27

Cs I and II Intensity Wavelength

250 2392.86 II 80 2414.89 II 25 2432.71 II 25 2443.24 II

130 2476.07 II 40 2480.41 II

130 2515.72 II 15 2523.66 II

130 2539.08 II 25 2539.17 II 60 25SO.65 II

130 2551.17 II 130 2568.17 II 2SO 2568.69 II

1000 2573.03 II 130 2574.54 II 130 2576.74 II 130 2590.09 II 250 2609.44 II 130 2616.27 II 25 2627.95 II 80 2637.14 II 25 2644.69 II

130 2648.07 II 200 2651.71 II

25 2660.24 II 130 2669.79 II

15 2671.17 II 40 2673.24 II

130 2686.60 II 25 2689.41 II 15 2701.19 ' II

130 2724.21 II 25 2730.07 11 25 2733.88 II

2SO 2748.23 II :'",J..

(" ' ..

80 60 80

2749.84 2757.81 2761.97

II II II

25 2766.10 II 250 2776.99 II 130 2788.24 II 130 2789.80 II 25 2793.32 II

130 2794.50 II 130 2799.41 II 500 2816.94 II 25 2820.27 II 25 2829.04 II 25 2829.42 II

130 2846.19 II 80 2852.42 II 80 2866.37 II

250 2881.19 II 25 2883.74 II 80 2899.75 II 80 2914.65 II

500 2931.09 II 500 2940.95 II 80 2942.25 II 30 2949.80 II 20 2968.38 II 25 2970.85 II

100 3001.27 II 80 3012.04 II 15 3020.37 II 15 3060.98 II

100 3066.60 II 25 3080.87 11

Intensity 100 80

800 800 500 100 800 100 100

15 15 15

100 15

100 15

350 500

15 ISOc 80c

1000 350 500 350 500 150 350 200 500 500

70 60

500 100 350 350

60 500 200 500 3SO 100 100

2000 1000 200 500

50 50

350 2000

500 400 100 200 350 350

1000 350 100 350 100 200 200 200 100

1000 200

Cs I and II Wavelength 3092.31 II 3180.94 II 3265.92 II 3267.13 II 3271.63 II 3329.43 II 3368.56 II 3559.80 II 3565.11 II 3651.07 II 3680.10 II 3687.64 II 3699.48 II 3732.54 II 3734.34 II 3751.40 II 3785.42 II 3805.10 II 3870.16 II 3876.143 I 3888.608 I 3896.98 II 3906.93 II 3925.58 II 3959.50 II 3965.19 II 3978.00 II 4047.18 II 4053.96 II 4067.96 II 4068.77 II 4073.36 II 4119.29 11 4121.21 II 4132.00 II 4151.27 II 4158.61 II 4193.20 II 4213.13 II 4221.12 II 4232.19 II 4234.41 II 4241.97 II 4271.74 II 4277.10 II 4288.35 II 4292.00 II 4300.64 II 4307.94 II 4327.58 II 4330.24 II 4363.28 II 4373.02 II 4384.43 II 4388.76 II 4396.91 II 4399.50 II 4403.85 II 4405.25 II 4410.21 II 4424.05 II 4435.71 II 4444.00 II 4453.44 II 4457.68 II 4459.18 II 4493.66 II 4501.S2 II 4506.71 II

Intensity 100 100 ISO

1000 800 400 c 150 150 200 c

2500 100 150 350 500 150 350 100 500 350 350 100 500 200 800 800 ". 800 500 800 500 500 100 400

1500 800 4()()

350 350 800 500

5 c 5

1000 11 30 c

8 350 500

5c 30

500 80c 30

4()()

120 15 50 c

100 15 15

200 200

30 200

35 200 c 100

c 100

c

Cs I and II Wavelength 4506.83 II 4515.50 II 4522.85 II 4526.72 II 4538.94 II 4555.276 I 4566.98 II 4571.79 II 4593.169 I 4603.76 II 4609.99 II 4616.13 II 4623.09 II 4646.51 II 4656.54 II 4670.28 II 4695.61 II 4701.79 II 4732.98 II 4739.66 II 4749.13 II 4763.62 II 4786.36 II 4830.16 II 4870.02 II 4952.84 II 4972.59 II 5043.80 II 5052.70 II 5059.87 II 5081.77 II 5096.60 11 5227.00 11 5249.37 11

: 5274.04 II 5306.61 II 5349.16 II 5370.97 II 5419.69 II 5465.944 I 5502.884 I 5563.02 II 5635.212 I 5664.ot8 I 5745.724 I 5814.18 II 5831.16 II 5838.835 I 5845.141 I 5925.65 II 6010.490 1 6034.089 1 6128.62 II 6213.100 1 6217.599 I 6354.555 1 6419.54 II 6431.969 I 6472.623 1 6495.53 II 6536.44 II 6586.02 1 6586.510 I 6628.660 1 6723.284 1 6824.652 1 6848.91 1 6870.455 I 6895.01 I

Cs I and II Intensity Wavelength

400 6955.52 Ik 200 6973.297 I

35 6983.491 I 200 7228.536 I 60 7279.90 I

200 7279.957 I ISO c 7608.903 I 300 7943.882 I 60s 7990.68 I

100 8015.724 I 60s 8053.35 I 60 8078.92 I

3SO 8079.033 I 1000 c 8521.122 I 200c 8761.415 I 600c 8943.46 I 350 9172.322 I 100 9208.538 I 3SO 10024.359 I 100 10123.414 I 400 10123.602 I 140 13588.31 I

15 13602.57 I 15 13758.83 I

350 14694.93 I 12 17923.62 I 9 17924.21 I

25 19162.53 I 20 19163.20 I SO 21311.46 I 45 21312.29 I

100 25763.49 I 90 25764.70 I

200 39421.22 I 180 39424.08 I

CsIII Ref. 78 - l.R.

Vacuum 75 556.91 III

250 584.15 III 50 584.40 III

1200 603.01 III 600 607.85 III 800 607.94 III

10000 614.01 III 1500 621.15 III 600 635.86 III 300 637.67 III

2000 638.17 III 25 657.94 III

450 663.82 III 450 664.60 III

2500 666.25 III 1800 673.06 III 400 679.60 III 800 687.55 III

5000 691.60 III 800 699.43 III

3500 703.89 III 1000 710.25 III

20000 721.79 III 20000 722.20 III

5000 731.S6 III 300 731.95 III

1000 P 736.66 III 12000 740.29 III

150 742.23 III 1000 749.94 III

, ..) 28

---

'\.....,.

GROTRIAN DIAGRAM FOR CESIUM

-9 2S1/2 ---- 8 2

p1/213/2

7201/213/2

5201/213/2

6201/213/2--.. , »>0 _. 7 2p112,3/2 r$f 11

6. 2P1/213/2--

-c Q)

a:::

IR 72S1/ 2 ,s

8 2 S1/ 2 --

W/7/7//7//7/7//77///7////7////////h6 28 1/ 2 ~./7//7/Z/.

Oprical and Quanrum Electronics 9 U 9 77) 509-518

Stimulated electronic Raman scattering in Cs vapour: a simple tunable laser system for the 2.7 to 3.5 Jlm region

D. COTTER, D. C. HANNA Department of Electronics, University ofSouthampton, Southampton S09 5NH, UK

Received 5 May 7977

Stimulated.electronic Raman scattering (SERS) in atomic vapours provides a simple method of extending the tuning ranges of pulsed dye lasers well into the infrared region. The special advantages of this technique in comparison with other types of tunable infrared lasers are discussed, and are illustrated by describing a SERS system which uses a modest nitrogen laser·pumped dye laser (- 20 kW). This produces infrared radiation tunable from 2.67 to 3.47 JJ.m by SERS in caesium vapour, which is contained in a heat pipe dven. Photon conversion efficiencies of up to 50% are obtained. The design of the heat pipe oven, operation of the system and optimization of experimental parameters are described in detail.

1. Introduction Within the last 10 years, the availability of rUl/able coherent light sources [I. 2) has made possible a number of new observations and techniques in atomic and molecular spectroscopy. These new possibilities stem from the hi.gh pO\'ier, spectral brightness, and directionality of the output from these sources. and in some cases, from a capability for short time·resolution. In many fields of appli· cation the impact of these devices is only begin. ning to be felt.

The most widely used and best developed of these sources is the dye laser, which is tunable throughout the visible region of the spectrum from the near ultraviolet to near infrared (approxi· mately 300 nm-l /lm) [3). Attempts to extend dye laser operation to longer wavelengths have not met with much success. This is an unfortunate limitation of dye lasers since the infrared region is of particular importance in molecular spectro· scopy. A number of different types of tunable illfrared laser now exist at various stages of develop· ment 11.2) and these are leading to many new applications such as remote detection of atmos· pheric pollutants, nonlinear molecular spectro· scopy. molecular super·excitation phenomena, photochemistry and isotope separation. Unfortu·

~ J9 77 Chapman and Hall Lid. Primed in Grea/ Bri/ain

nate]y most of these tunable infrared lasers have disadvantages of complexity and high cost. How. ever an exception to this is a recently developed method of infrared generation based on stimu· lated electronic Raman scattering (SERS) in atomic vapours [4-7). This nonlinear optical process has been used to convert the output from pulsed dye lasers operating in the blue and near UV region into a tunable source of infrared radio ation. For the many laboratories which are already equipped with suitable dye lasers: the SERS

------7;>J'2

____w.Ly_-~s-------_-_--7P,,;:

---'~--5s

Figure 1 Energy level diagram of an alkali atom showing the SERS scheme for IOf~ared generation. ('The different levels have been given labels specific to Gs- 75 scattering in caesium).

509

. .; . .

...... ~ a

I

D. COlfer. D. C. Hann."

method provides an exceptionally simple and cheap method of extending the wavelength cover· age of these lasers i;1to regions of the medium infrared range.

The basic SERS scheme is illustrated in the elec· tronic energy level diagram of Fig. I. where an alkali atomic system has been assumed. The intense dye laser (pump) light is used to excite a Raman transition between the electronic ground state and an excited s state. In this way Raman shifted (Stokes) radiation is produced at the frequency W = w p - n fg • where w p is the pump frequency s and M1fg is the energy of the electronic Raman transition [8]. For SERS in alkali metal vapours this Raman shift is large (20000-30000 cm- 1 ) allowing infrared Stokes wavelengths to be gener· ated directly from a visible or near L"V dye laser output. Since the Raman shift is constant, as the pump frequency is tuned the generated Stokes fre· quency will follow and so SERS provides a method for producing a !Unable infrared output, Furthermore this Raman generation process can be made efficient by resonance enhancement, This is obtained by tuning w p close to a single· photon resonance between the ground state and the doublet p levels. This produces a large Raman gain for Stokes scattering to the final s level having the same principal quantum number as the inter· mediate p level [6].

Experimentally the SERS method is very straightforward: the pulsed dye laser beam is simply focussed into a heated cell containing the alkali vapour: and the infrared radiation emerges in a narrow collimated beam collinearly with the incident laser beam. The only components that . are required in addition to the pulsed dye laser system are the vapour cell and an optical filter to discriminate between the SERS output and other radiation emerging from the atomic vapour.

We describe here an infrared source based on SERS in caesium vapour which is pumped by a dye laser of modest peak power (- 20 kW). In this particular case the 6s ~ 7s SERS transition

53'1'1 ~~ (Raman shift n lg = 18536 em -1) is used to produce infrared radiation tunable over the wave· length range 2.7-3.5 pm and with an output power of up to I kW over a significant part of the tuning range. The aim of this paper is to pro· vide details of the practical aspects of the con· struction and operation of the source and the

-") 510

optimization of experimental parameters. We also wish to indicate the special advantages. as well as some disadvantages. of this technique for infrared generation compared with other methods.

2. Vapour cell The caesium vapour. at pressures of I-3D torr, is contained in a heat pipe oven. In such a cell the alkali met:1! is constantly recirculated by capiJlar~. action through a mesh "wick' (see Fig. 2), thus allowing operation over long periods of time with· out the usual problems caused by the metal depositing out at the cold end windows. For a detailed explanation of the operating principles of heat pipe ovens, the reader is referred to a descriplion by the inventors. Vidal and Cooper [9]. Fig. 2 shows an oven which we have found suitable for use with caesium vapour. The end windows are entirely separated from the vapour by an inert buffer gas (argon). and are kept al room temperature by means of the water cooling coils. In Ihis W3Y Ihe windows can be all3ched 10 the ends of the oven body usin~ ordinary rubber a.ring vacuuill seals. In the present case. windows made of fused silic3 (InfrasiJ) have been used, although we have also used caesium iodide and calcium fluoride windows when generating longer infrared wavelengths [6]. Another special property of heat pipes is that very uniform vapour den· silies can be maintained over long distances, which may be necessary for optimum phase· matching in parametric conversion processes such as third harmonic generation [10]. However in SERS there is no phase·matching condition to be

':) ../fer 9:::5 I I

water cooling I~' nec:er coils oss~mbly ,

--25c:-:,s

--------65c1'\S , no. 3D!.. g'cde slc'r'iess s~

Stimulated electronic Raman scatterillg ill Cs I'apour//lila

tact with air, it must be treated with some caution. " ~_....

with the initial alignment of the laser. a diffraction·

Jso :.IS

ed

:s

Ir)'

th

:0

~r

·ny

satisified. and this property of heat pipe ovens is not important.

In the oven shown in Fig. 2. there is a length of about 9 cm between the heated section and each of the copper coiling coils. In an earlier design. this length was much shorter. as little as 1-2 cm. but this caused some difficulties as the caesium tended to solidify and accumulate in the cooled regions and was then prevented from !lowing as a liqUid back to the central heated region. As well as depleting the vapour. this accumulation of solid material was sufficient to partially obscure the light beams after a few weeks of operation. However. the later design appears to be free from this problem. An oven of this type, used with caesium at pressures of 1-30 torr. has given entirely trouble free operation and has been in use for more than 350 hours over a period of 8 months including more than 50 heating/cooling cycles. There have been no signs of contamination or fogging of the end windows by the alkali vapour.

The interior surface of the steel tube was carefully cleaned by scouring the inside with damp cotton wadding and carborundum powder. The tubing was then washed in water. rinsed in aqua regia. washed again and dried. The heat pipe wick consists of 5 or 6 turns of stainless steel mesh ( lOO mesh, grade 304 stainless steel plain weave gauze, 41 swg) and is a tight fit inside the steel tube ext~nding the full length between the end windows. Before insertion the mesh was cleaned in a similar manner to that above. Finally it is. of course, necessary to ensure that the completed construction is vacuum tight in order to avoid gradual oxidation of the alkali metal.

The central 25 cm of the tube was wrapped with an electrical heating tape having a quartz fabric insulation [11]. Silica tape was used to hold the heater in place, and then over.wrapped with a layer 3-4 cm thick of ceramic fibre blanket [12] (See Fig. 2). Finally the whole heating assembly was secured with glass tape. With this construction about 60-90 \V of electrical heating is sufficient for 1-30 torr operation (280-4300 C) using caesium.

The oven is loaded with about 109 of the alkali metal, which is' normally obtained from the suppliers packed in a glass breakseal ampoule. Since caesium ignites and sputters Violently in con·

A con\'enient method of loading the metal is to fill the oven. yet unheated. with an inert buffer gas (for example argon) and maintain a small positive pressure so that with the end windows removed there is an outflow of gas. The glass ampoule. clean and intact. can be introduced into the tube and then. when positioned centrally, the tip of the ampoule can be broken (using a glass rod for example). The end windows are then replaced, the pressure of argon in the oven reduced to around 10 torr. and the oven heater switched on for about :2 hours in order to vapourize the caesium from the ampoule. When the oven has been allowed to cool again to room temperature. the argon pressure can be increased once more to slightly greater than atmospheric pressure before the end windows are taken off so that the used ampoule can be removed carefully.

The procedure for starting up the oven is to set the required pressure of argon and switch on the heater. \\'ith our particular arrangement about I hour is required for warm up. lf the volume of the interconnecting vacuum pipes and gauges is reasonably large compared with the volume of the heated zone in the oven. then the equilibrium vapour pressure will be only slightly greater than the initial pressure of the cold buffer gas. Our usual practice has been to switch off the heat pipe at the end of each day of use and restart it again as required.

3. Pump source The dye laser. which is of our own construction. is pumped by 200 kW of output from a Molectron UV·300 nitrogen laser. It is essentially similar to the well·known design by Hansch [13]. except that the beam·ending telescope has been replaced by a prism [14 J. Tuning is by means of a halo· graphic grating used in first order, and automatic wavelength scanning is achieved using a stepping motor drive to rotate the grating mount. Only one dye, a solution of 7.diethylamino-4·methyl coumarin in methanol. is required to cover 449467 nm. This is the range of pump wavelengths for which this SERS source produces an infrared output using the 65 --+ 7s Raman transition in caesium. With this dye an output energy of typically 140 J.1.J in a 7 ns pulse (20 kW peak power) is obtained from the dye laser. By taking a little care

D. COffer. D. C. Hanna

limited output beam is obtained with nearlyGaussian intensity profiles in the vertical and horizontal directions_ The spectral width of the output obtained from this laser is 0.1 em-I.

4. SERS performance We have found that the best performance in terms of infrared tuning range and output energy is obtained with a caesium vapour pressure of 10 torr and with the dye laser focussed at the centre of the vapour cell such that the confocal parameter bp is approximately equal to the length lli the vapour column L (L/b p ~ I, L = ~S-30 cm). The SERS output energy obtained under these conditions is shown in Fig. 3 as a function of dye laser tuning. The highest output energies of 7-8 pl represent photon conversion efficiencies of up to 50'1. (These measurements were made using a Laser Precision pyroelectric energy meter. and due allowance was given for the optical losses in the vapour cell windows and Ge filter.) Observations using a fast InAs detector indicate a SERS pulse length of - 5 ns. implying a peak output power of - 1.5 kW.

There are three distinct minima in the SERS output energy curve shown in Fig. 3. The outer two occur when the dye laser corresponds in frequency with the resonance doublet 6s-7P!'2.3,2' and are accOinpanied by weak transmission of the dye laser beam through the vapour. These minima are therefore probably due to single-photon absorption of the pump radiation as well as losses due to other resonantly enhanced multi-photon processes. The central minimum at - 21855 cm- I

was not accompanied by any observable dip in

Stokes frequency W !em-')s 3000 3200 3'00 3&00

.tran~mi~sion of the dye laser beam. Its origin.is not known for certain. although we suspect that it may be due to a single-photon excited state absorption of the SERS radiation [15].

SERS output energies greater than 30 nl (- lOW) were obtained for a continuous range pump frequencies between ~ I 4~0 and 22 ~;O ':i. giving a tunable infrared output over the 860cm- 1

range 2.67-3.47 pm. This arbitrary criterion for defining the extent of the tuning range has been selected for two pract ical reasons. Firstly the detected energy of 50 nl gives a signal-to-noise ratio of - 100 when using a typical room temperature pyroelectric detector. Secondly. beyond the tuning range indicated there is a large pulse-topulse variation in the output energy. Whereas at the centre of the tuning range the infrared outpui amplitude stability is - ± 5"; (approxiI113tely the same as that of the dye laser). at - 80 cm -I from th'e edge of the tuning range (as defined above) the amplitude stability is - ± 20~"'. and at the edge of the range it is - ± SO

L /bp =5

" , /' -~.., r ::Jp = ,',j'. I' 9 • .i\

! r· I ~ i\'0' f ~~/ t f If I

8r-"""--~----'------'---r--'

6

fJ J I. 2

ClAoo

1

I Sril1ll1laTcJ clccrrc II/ic D. CoTTer. D. C. Hallna

this absorption can become saturated. and the total absorption loss decreases as the volume of the laser beam in tIle vapour is red uced. This volume is minimized when L/b p = Y3. In that case less than 107£ of a 140 111 laser pulse is absorbed over a . :::!5 cm vapour column at 10 torr.

In Fig. 4 it can be seen that a value for L/b p of - 0.3 or greater is necessary to obtain continuous tuning between the 6s-7Pl ~.3 ~ doublet. As mentioned earlier. the best results in terms of tuning range and output energy are obtained with approximately confocal focussing of the pump beam (L/b p ::::: I). By focussing more strongly (L/b p ::::: 6). the peak output energies are slightly lower and there is a more rapid fall off in the wings of the tuning. curve.

Using confocal focussing. SERS tuning range measurements have been made with vapour pressures of 1,3,10 and 30 torr. and these are shown in Fig. S. At I torr the effects of the inequality of the doublet oscillator strengths are very obvious. The ratios f6s-iP3" If6s-iP) , and J~s-7p) ,I 17s-ip I"~ are each about :. and it can be dearly seen in Fig. 5 that this results in the tuning range

:_,.~ around the 6s-7P3:2 resonance being nearly twice . as wide as that around the 6s-7PI'~ resonance. At

the same time the minimum Raman output energy with the pump tuned very close to the P3 ~ reson· ance is about half that obtained close to the PI ~ resonance. while the width of the dip in l)utput energy is also greater around the P3 2 level. So whereas the'tuning range is greater in the region of

;'0;.

t;,1 ,

,9, <

9 ' ; :

; < -.,j 1 ~...~ i ~ ~-

'" Figure 5 Infrared output energy as a function of dye laser tuning for different caesium vapour pressures.

~'4 ,

the stronger resonance. the effects of the resonant competing processes are also more pn)nounced.

The best performance has been obtained at 10 torr. Increasing the pressure to 30 torr has little effect on the tuning range and a,tually reduces the peak output energy by a factor of - 2. Evidence points \0 this being due to the greater absorption of the pump radiation by Cs~ molecules at the hig.her pressure [1 i\].

With the dye laser beam focussed confocally (L/b p =:: I), the divergence of the generated SERS radiation. as measured by scanning a narrow slit across the beam in the far field. is 6-S mraJ (full angle to half maximum intensity points I. This divergence angle does not appear to vary over the tuning range. and is in very good agreement with the calculated angle of 6 mrad assuming that the infrared beam is diffraction·limited and emerges from a beam waist equal to that of the rump. We have also monitored the divergence oi the rl'siJu~I' dye laser radiatioll emerging from the hl?at pipe. and througl1\lut the lUning range there is no evidence for any induced focussing or Jt>Cocussing of the pump beam.

In stimulated Raman scattering. Stokes radiation travelling in both the forward and backward directions (parallel and antiparallel with the incident pump beam) is generated simultaneously. In the infrared source described here we hal'e found that the maximum energy in the emergent backward Stokes wave is - 2.5 times smaller than that in the forward wave. This is in good agreement

• r- ,-- -,---,_....,- ---,

8

t-' J L 2

\\'ith the result of 3 11\ saturation pro.;esses it d\'Jlamic cllmpetitillil b~,k\\'ard \\'aves [I g J that the useful (> 50 tuning range is - 20r~

I The Spl'ct ral line\\'

radiati{\[1 (in the rOI'\\ III ined using a I m f/ 1

I

,hrllma'lOr (instrumeJ found tl) be in the ral this range the line\\'id the vapour pressure i~ al so as the dye laser i mediate -;p resonance amounts 10 no more case. These infrarl'd 1 0'1' magnit udl' greater the two-photon Ram and also 2.s times gr( 1he pump source (0.11 I?xpl?,teJ the generat ~omewha t narro\\'cr 1 viTI ue of gain narro\\ the case suggests tha cesses are occurring. factors inlluencing tl plete at present and invesliga tions.

Finally. we have: Stokes radiation. Us laser beam. over mo generated Stokes be the same direction. pump frequencies b and a point nearly r doublet. the infrare polarized orthogon~ more detailed descr explanation is given

The perfonnanc can therefore be su

I ;idyl' leser i

I I

, ~2 leser

S;ill1//lateJ electronic Raman scattering in Cs l'a/lOl"

II IIh the result of a numerical calculation of the ~J Iur:nion processes in SERS. which includes the ,lyn:lInic competition between the forward and h,kward waves [19J. In addition it is observed thai the useful (> 50 nJ output) backward wave luning range is - 209C smaller.

The spectrallinewidth of the generated Stokes raJiation (in the forward direction) has heen deter· min~d using aIm I/1 0 infrared grating mono,'11I"1lmator (instrumental resolution 0.1 cm -1) and ]',lunJ to be in the range 0.25-0.55 cm- I . Within Ihis range the linewidth is observed to increase as th~ \'apour pressure is raised from I to 30 torr and J!SI' as the dye laser is tuned closer to the intermeJiate 7p resonances, although the total variation amuunts to no more than a factor of two in each cas~, These infrared linewidths are about an order "I' magnitude greater than the Doppler width of til~ two-photon Raman transition (- 0.03 cm -I) alh! also 2.5 times greater than the linewidth of the pump source (0.1 cm- I ). In fact one may have ;:,?~cted the generated infrared linewidth to be ,":~le\\'hat narrower than the pump linewidth by ,in U~ of gain narrowing. Tile iact that this is not till' case suggests that some line-broadening'proc~ss~s are occurring. Our understanding of the (a,tors inl1uencing the SERS line\\'idth are incomplete at present and is the subject of continuing inve stiga tion s.

Finally. we have studied the polarization of the Stokes radiation. Using a linearly polarized dye laSCT beam, over'most of the tuning range the

The performance of this t unable infrared source ":JIl therefore be summarized briel1y as in Table I.

TAB lEI Summary of performance

Peak OUIPUl encr~y R/JJ Pulse dur~lion 5 -6 ns Peak 1'0\\ er J ,5 kW

Tunin~ rante 2.67-3.4 7/J1l1" Spectralll'idlh (1.3-0.5 em" OUtput heam di"ertenee 6-8 Illrad. diffraction-limited Repetiti,'n r~te 4 Hzb

Amplitude ~tahilit), ~ 5' (.c = 1'\('1' ,d

"Continuousl) lunable Ol'er Ihis 860cm'I rante lI'ith > .50nJ tIO\\') output.

b Limited by heating in the dye laser ,'ell. PotentiallY could be inereas"d 10 50 Hz (ma:\imum rcpl'liti"n rale of nitr'lgen bserl by incorpor~ling ~ stirred or ilLl\\'ing d)'e solution.

C AI c,'ntre of llInin~ r3n~e, d .-\t edge of derined-Iuni'ng range,

5. Applications In ordl'r to demonstrate its simplicity (If (lp~ration. the tunable Sllurce has been used to obtain infrareJ absorption spectra, The dual heam ratiometer arrangement shown in Fig. 6 was used. Frequ~ncy scanning was obtained by tuning the dye laser using a stepper motor drive to rotate the diffraction grating. Fig.7 is an ahsorption spectrum of a sample of ~orth Sea gas showing a part of the R·branch of the CH 4 fundan1ental band around 3000 cm -I . This spectrum was obtained with a 4 Hz laser repetition rate. - 3 cm -] !minute frequency scanning rate and 3s time constants

, in each detector channel. Thus the time required to scan the 80 cm -I range shown in Fig. 7 was about ~5 minut~s. By increasing the repetition rate to 50 Hz (see Tahle II. this scan rate could be reduced to 2 minutes.

Fig. 8 shows an absorption spectrum of ammonia gas. in which the principal Jines are fundamental VI R-branch doublets. A large num· ber of additional lines are visible particularly in

-------,

_:,-,: ':ser ;

,. - ..._---------,

:ser : "'-~'

Figure 6 Dual beam spectrometer arrar'\gement. The electrIcal signals from the two pyroelectrIC detectors are ratioed and the result displayed

on a chart recorder.

I

Still/Illatcd citD. Correr, D. C. Halllla

,.

, J,{:, '

!.( t '. J," " 1,;. .,"'y ,; ',. ",~,.. ,. Figure 7 Absorption spectrum 01\,

'

North Sea gas (100torr. 1S' C, 7 cm length) showing a part of theCHI. CH. R·branch, The open circles

111111111111111111111111111111111111111111111111111' 1111I11111111 . . - _. - . ._. denote prominent water vapouri I

3~58 311.8 312) absorption lines.

the region of the V J R(4) doublet. and these are due to absorption by the overlapping fundamental V3 band and also water vapour. The VI R(5) lines provide a standard resolution test. These peaKs are 1.8 em -I apart and numerical calculations shows that the degree of resolution shown in Fig. 8 implies an overall spectrometer resolution of 0.7 em -) . This figure is within a factor of 2 of the measured SERS linewidth.

The observed resolution is similar to that available using a conventional grating monochromator. Thus, although the SERS source in its present form does not offer advantages of high resolution, we have at least shown that spectra comparable in qualily to those obtained by conventional means can be obtained by using the SERS source in a very straightforward fashion. The advantages 10 be gained from using the SERS source lie in its brightness since this will allow

RIl.l

3~8J 3~SO

Figy,e 8 Absorption spectrum of ammonia gas (100 torr, 15° C, 7 cm length). The resolution of the v, R(S) doublet by increasing the length and diameter of the heat indicates an overall spectr-"rnetl!rres()LuJignQffr2mL". . __pipeoven-wi Iha-corrc.spond ing-in Grease-in-beam

516

spectra 10 bc ubtained in situations for which thermal sources are wo weak (e.g. small samples. samples in inaccessible locations. etc.) Also the Iinewidths obtainable are cenainly adequate for a number of applications involving sckctive vibrationJ} excitJtion of molecular spe,ies. The short duration of the SERS pulse offers the further possibility of spectroscopic measuremenlS with a time resolution of a few nanoseconds.

6. Discussion We have described in some detail the construction and behaviour of a tunable infrared source based on SERS in caesium vapour. Compared with other tunable infrared lasers [1,2], the SERS scheme offers outstanding advantages of simplicity in its const ruction and operation.

By using an atomic vapour as the nonlinear medium. many of the difficulties associated with crystals are avoided. There are no problems of commercial availability, poor optical quality or susceptibility to permanent laser·induced damage. Al the same time a suitable heat pipe oven can be very easily built in the laboratory at low cost and withoul the need for special facililies or techniques. The reliability of Ihe whole system should be deter· mined largely by the reliability of the dye laser and its excitation source. An addilional important feature of the SERS method is its scalability, making it attractive for high energy and power applications. B.\' using a high. power pump source. and

si/e. 1l1u-:h gTi [5, 7].

Since the ;; de\'icr f,lr pI',' shift. all the \' fa,ilities 3rC r extra infrared n(1 Jdditil1nJI would increJ$ bscr system. desired. be CJ Tile infrJred r c'olincarly wit simplifies the fuJlo\\'ing the c'ritical1y cont nlJlching 11r b ary. Results p llutput is rebl the JlbJi \'Jp' ,'onlrlll of Ih, In addition tl' alignment. fir lUning of mal precision can currents. The plexity and c

Each one source now a

TAB L E II SI nitrogen laser·p

,\Ibli melal

10:"

('s"

ill

• S.rimu!aTcd elecTronic Raman scal1crill.!! ill Cs I'G1J(llir

i I j

'j I

size. much greater output energies arc possible and demcrits \vhich mean that it will not be thc [5. 7] .. most effective or appropriate choice for cvcry

Since the atomic vapour is used simply as a application. This is also true ofSEKS. which has dC\'ice for producing a fixed optical frequency two primary limitations. Firstly. the infrared shift, all the wavelength selection and tuning lincwidths that have been obtained at prcscnt facilities are provided by the dye laser. Thus the (0.3-0.5 cm-') are about an order of mag.nitude ext ra in frared capability offcred by SE RS requires greater than those obtainable using pUlscd semi· no additional wavelength control functions which conductor diodc lasers and spin.tlip Raman would increase the compleXity of an existing dye lasers. and 3-4 orders orl113gnitudc gre:J(cr than laser system. Frequency monitoring can. if ·those obtained with CW semiconductor diodc desired. be carried out at the dye laser wavelengths. lasers. spin·flip Raman lasers and threc·wavc The infrared radiation emcrges frulll the vapour diffcrence mixing. Our understanding of thc colinearly with the dye laser beam. and lhis greatly factors influcncing the SERS linc\\'idths is incomsimplifies the alignment of any optical compollents plcte at present and further work is rcquircd. following the vapour cell. With SERS there are no but it is unlikely that in this rcspect at least SERS critically controlled parameters and no phase· will ever compete wit]l these other types of tun· matching or beam overlap adjustments are necess· able lasers. Sccondly it appears unlik,:,ly :Jt present ary. Results presented here show th3t the SERS that complete coverage of the mcJium infrarcd output is rel3tively insensitive to the conditions of range will be possible with SERS by using only the alkali vapour. so there is no need for precise one atomic vapour. Howevcr. cI'cn with thcsc two control of the temperature or buffer gas pressure. limitations. thc SERS source is still attractive for In addition there are no requirements for ca\'ity a great many spectroscopic applications. alignment. fine angular adjustments. synchroJlous In addition there are many opportunities for tuning of many optical components. cryogenics. or further development Table" contains a list or precision controlh;d magnetic fields or electric the electronic Raman transitions that can be currents. These requirements add to the com· reached using thc fundalllen tal frequencies of plexity and cost of other infrared sources. nitrogen laser·pumped dyc lasers. Many more

Each one of the different types of infrared transitions can be reached by frequcncy·doubled source now available has its own special merits dye laser outputs. ane' these include transiti.ons . TAB LEI I Summary of SERS transitions in alkali metal vapours accessible using the fundamental frequency of

nitrogen laser·pumped dye lasers (Frequencies in em")

Alkali SLRS metal lr~nsilion

1Raman shifl in hrackets ),

\0.:" 4s 50s t~1 O~7)

Rh'o 5s 6s

,>' , ", ,

" ".,

)~ (II \ '

• ,

, r

.

D. Coner, D. C. Hanna

to atomic Rydberg states which offer the opportunity of generating radi:ltion over wide. ranges of far infrared wavelengths. Future rossi· bili~ies include the use of shorter wavelength dye lasers [23]. and also the use of number of Raman transitions in alkaline earths which are as yet largely unexplored [5]. \10re Sopilisticated tech· niques could include the use of multi·pass and resonator configurations. although so far our preliminary experiments ..... ith a resonator have nut proved very promising [24). Finally it should be possible to considerably extend the range of infrared wavelengths by using transitions from pre· vious!y populated excited states [25.26].

However despite the many possibilities for greater sophistication. we believe that the simple basic source as described in this paper has a great deal to offer since it provides a very straight. forward means of extending the wavelength cover· age of existing dye laser systems.

., Acknowledgements "(. Thanks are due to Drs T.R. Gilson and J. Black for ;"t

valuable help with the cunstructiun uf the heat pipe ovens. and to R. Baker for te.:hni.::al assistance with the dual beam ratiometer. This work has been supported by the Paul Instrument Fund and the Science Research Council. D. Cotter holds an 1851 Exhibition Research Fellowship.

References I. .\1. J. COllES ~nd C. R. 1'11)(;[0:". Rep. Prog.

Phys. 38 (1975) 329-460. 2. E. n. HIN"lEY. ". w. :"Ill ~nd I ..~. Rlt.:.\1. in

'Laser Spectroscopy or .-\ lOms and .\Iolecules'. edited hy H. \\'~Ither (Sprin~er-\'erb!=. Berlin. 19761. Topics ill ,1pplicd Physics. 2.

3. F. 1'. SCHAFER. led). 'Dye L~sers' (Sprin!=er-\'erbl!. Berlin. 197J). Topics ill Applied Physics. 1.

4. 1'. P. S() R()" I" . J. J . \I , '\ '\ r ~ nd J. R. L.~ \' ".~ RD. AI'PI.I'h.,·s. 221 197.'1 34~-4.

5. J. L. C.·\RlS'I I \. and 1'. C. Ill''':''. ()pI. CO/ll III1111. 14119751 8-\~.

6. D.l'IITI I R.II.l'. H.~"\'.~.I'.~. K.\I("".\I\'L:" ~nd R. \1"'\ ..\TT.ihid 15 119-51143-6,

7. n. CfiT 11 R. I I. l'. H ~ ,,\' .\ ~nd R. \\'Y~ TT . ihid 16 (1976) ~56-8.

R. \:pr ~ rcvi",' of qim\l!:ll"d Raman sc~t1erin~ and i" ~pplic'~ti"lls. ,,"''\1. \1.~11 R..~/)pl. Phys. 11 t19i6, ~09-31.

9. C. K. \·ID.·\l and J. COOI'ER.J. .·lppl. Phys. 40. 119(9) 3370-4.

10. R. R. .\lllES ~nd S. E. H.·\RRIS.II:l:TJ. QUOIII. Decr. QE·9 119'7:;) 470- f;4.

11. Uectrt>lhermal Fn!=inc,'rill~ Limited. ~7'.1 ,\e\'illc Ro~d. L,'ndon I: 7 9Q\': quartz fabrie Insulation healing t~pe HT550 (ROO' (' m~.\. ~(Jn\\'l.

12. \10r~~nitc ('er~mic !'ibrL's Limi1l'd. \'e,t,'n. \\·irr~1. \lcr;eY5idc. [ngl~nd: Trit

A Seat-of-the-Pants Approach to RIS Theory

William M. Fairbank, Jr. Physics Department

Colorado State University Fort Collins CO 80523

OUTLINE

Resonant excitation - selection rules, saturation

Ionization - rules of thumb for cross sections, saturation

Isotopic biases in RIS - laser mode structure, m-states

Vaporization - thermal, sputtering, ablation, efficiency

ENERGY LEVEL TABLES Charlotte E. Moore, Atomic Energy Levels, Vols. 1,2,3, NSRDS·NBS·35

Wiese and Martin, Atomic Energy Levels' the Rare Earths, NSRDS·NBS-

Additional tables 01 selected elements published in J. Phys. Chem. ReI. Data

RIS Data Sheets' E. B. Saloman, Spectrochimica Acta 45B, 37 (1990), 46B, 319 (1991) and 478,517 (1992).

Actinides and lanthanides· articles by E. F. Worden, R. W. Solarz, et al. LLNL

hltp:/laeidata.phy.nisl.gov/archive/data.htmi (NIST atomic data tables on WEB-data for 37 elements)

hltp:/lplasma-gate.weizmann.ac.iI/APl.html (list of other sites having atomic data)

Chapter 8 of Atomic and Molecular Data for Space Astronomy, P.L. Smith and W. L. Wiese, Eds., No. 407 of "Lecure Notes in Physics", Springer-Verlag, 1992. (review of data sources)

E (em-1 )=lIA(vaeuum in em) A(air)=A(vae)/n(air) n(air);:::1.000277

NIST ENERGY LEVEL TABLES ON THE WORLD-WIDE WEB

http://aeldata.phy.nist.gov/archive/data.html

;1'1," l,·rll , ' ; I I ~, , I l.I·':" .:,,'.: :Ii .. 10. " lI' I ,~ I , " , " .',;,,' I H, ",. ,t", , ~ ,.. I ~. I • ., , \" .. I l'~. I'l- , l'n ..ll·~ H

I ~ 'I' " f· ... \ oIl-'! ':1 1·110 1.:1"1 ~'114 ;,,', 1·,',

SELECTION RULES

Quantum numbers: (vector additon)

Single electron - n, I, ml, s=1/2, ms, J, mJ All electrons - L, 5, J=L+S, mJ COITlplete atom - J, I, F, mF

Electric Dipole selection rules:

Parity: p = (-1)~:I i ; odd-even only ~J=O,+1 (0+0); ~mJ=0,+1 (0+0 for ~J=O) ~F=O,+1 (0+0); ~mF=O,+1 (0+0 for ~F=O) ~S=O (rigid low Z; broken high Z) one electron changes (broken-complex atoms) no ~L rule!

EXAMPLE-Sn Nd:YAG based system: easy lasing 540-740nm with non-carcinogenic dyes

Frequency doubled output: 270-370nm (27000-37000 cm-1) Frequency doubled and mixed output: 220-265nm (37000-45000 cm-1)

I. 1'. ; .:I·I~ "olls

Sn I Sn .. .0.

i ),....... J: 1.-\'1 ; 11l1"n":,1 I 111.~ 0) , I">llll~. : I I, •. It/:. l,f . 1111.",",,1 : I Ill.... !I

. -_ ..--.... - . --_._ ... _.. . - -._--_.._--- _.. 110"":1 ~ !II If J !li'.II." i Irl;,u'.!

-~

i~ :,,' ftpl11·i .... hp I 1;1' q." I ~l;~~ ; 1:'3~ \1 f I -4:,:.! II :u' [".\.1';",.(,1' ! t.,.-s ,) 1t;~I;Hi 3~ I

I II!.'!, :,.~I !'I,iJ I'i.. ,:,J :..1 III" II ~III•. 1.\. I II II :: :,,,J !",I~J·i •• ·hp I "" 'S [I 11.1' ; :u l !l/H'I':...h,' 1." IIJ !:!

•... '::1 ~ : ;\; ... Ii: : ~~lll l: :.~I !'P111~; .. ,;> ;. II'~ i III 1 i.~.~:~.~: j'

1 I ~ I :~ :._,' !I"PI',.,.:'. ,. I ,;.:: I,; ..••. :, '" :I·i.~"'· ... 'I'. I

,; :"'!J/,·ll},.:,.1 :.. 1 'I'"~ ! ~ I :,. :.,,1 . I i:~;~:~:::: f

I u ~.,~,'i: ...' ,.,. II' ! 1:11.11111

·1~;II: ;; :.1 'I'" I J 1;::.1:, :!

:,·1 IJ ,.. I :.01 .'.~ :'II .", 'It 1.1.1', .. :, . I ~~l ....

1:1;'::b ~ ~" '" I II t ~:IW~), ~ ",.. ".~ .. I':.... "" 1;11111, ~

;l;h"i I ;, " :"1. -I' ~ '. ;" I ~ I !l:-d )0.1; ~I

, .,,~,;~u S .•"t :~.,.; :

."t.",.. .:., ,~

:.,,1 'II •

:, .. :".,·":.,'111

-" -';1·'1'::... 1:.. / ',f I) , :,.·:".,-l'j.,I:..1

______ ~ :.L_

----------- ------ --- ----- - ------- ---

TRANSITION PROBABILITY TABLES W. L. Wiese et aI., Atomic Transition Probabilities Vol. I, II NSRDS-NBS4,22

(quite complete for Z:::1-20)

J. Reader et al. , Wavelengths and Transition Probabilities for Atoms and Atomic Ions, NSRDSNBS 68 (also in Handbook of Chemistry and Physics (principal visible lines of most elements)

C. H. Corliss and W. R. Bozman, Exp!'lrimental Transition Probabilities for Sp!'lctral Lines of S!'lv!'lnty El!'lm!'lnts, NBS Monograph 53 (old, inacurate, but useful for rough estimates)

RIS Data Sheets· E. B. Saloman, Spectrochimica Acta 45B, 37 (1990). 46B, 319 (1991) and 47B. 517 (1992).

http://aeldata.phy.nist.gov/archive/data.html(NIST transition probability tables on WEB-8 elements)

http://plasma-gate.weizmann.ac.il/APl.html(list of other sites having atomic data)

Chapter 8 of Atomic and Molecular Data for Space Astronomy, P.L. Smith and W. L. Wiese, Eds., No. 407 of "Lecure Notes in Physics", Springer-Verlag, 1992. (review of data sources)

NIST TRANSITION PROBABILITY TABLES ON THE WEB

http://aeldata.phy.nist.gov/archive/data.html

Ul: .. ..:.:.....,~~ • .:. ".1"... ::.1." :h~ •.., L'.. :'"L',;h;''':. .;a. .:.M.IJ ..;~ >I~ \ .• .1; •.L..l~I.o:';'J,.·• .;. ..... l. .. ~.I~~yJ. ..

I •. -~;..;n THIH· :7','1_ ,i •.·: .•.• , , ~ II 1'1. ~ , I •. :,") !.UI I.·· ..;U· :.):" .. ; . 1111.:. I , ; .. 11.·-1 ·11 1"" I :;,'II).l: ., , , , , , ,I 0.1 1",1' , \1.: .. ~bt.b" a,'" , "II ",I" ..." ..'bOI)I) , , . " , , aII, ,:; 'I" .) ,.".,1" ." , ". I)"' '·1." '1'01 • ", , IJ.: ~"4.1 ~ , , 01' ",II , I. '" " oo..'t1 ., " 0" II, I 1::, '.1.' 1"8 ~, ., .:,;:1 to :1 .. ~ .. , , It ,, , . ' .. , ~. t .,1 • · , ~'b ,'I', U · ... , II';: , .:' ... .; ··0, ':, , ., , .. " , ,'1\ '1\ , .. , ,t '.' ItI.',1 ", , .. : ';:1 ~.lI(.o 0 01.1.:" , " 0 0(.: ., ..- ," II, I 1:1 · "

nl" " ,I'''' .:tlOt.lI ·"·0 OO,a •, ,, " 0 0" [ H' I [, · .. "'i' II".: ;..' J t> :.~ ;0' , , B \',1, , 0 " 0 0(.0'1.: . ,. .:".. n·

'1\ I ., · JI ·~,.; , 1'

i.'-.._

EXCITATION RATES AND CROSS SECTIONS

Excitation rate:

W _ ( ) I g2 A3A 2l12 - g V -----==-=e g1 81th

g(V ) =~ 41112 ~ a 1t D.V Gaussian

2g(vo) = -

1tD.V Lorentzian

Cross section:

SATURATION OF AN EXCITATION TRANSITION

Cross section estimate

Saturation intensity (two-level system)

(l 0-33 J5)(10 10 en1)(1 08 /5) ______5..,....."..-------:_ :: 10 W

(10-4em)(lO-12eln2) em

Saturated excited state population fraction Typical power: 1 MW easy to saturate!

--12

2

I

I ,,, ,."..u.u"'I.d."", .. I EXAMPLE-SnI I""" rur'.,:' ...·'It .... 1'''1''.11'''' ~ .... 1"",,,1, 1·...1...10,1,1''''or' .

'''11\11 11.1'''' II ":,,,:, 1\1111" .. '11

-u I .!1l~.1 I UI.!;,!~ Ulh II :!I'I".' ~;Ilh :!" .!:!(tl'.7 ·Ut";,, ...." II :!:.!U, I 1-1;,11" II t. II S."III"''''N

" '10

:!:!hH" a:4.XH I=! I' ~:!lIh 1 1:1·k', II I·

III :)U:.!:J:J

4:l7UIl

E e

'" oD ."...

E e

oD '" ~

II

,,:,U ~I :!:\I':' ~ .".1 :~.;, :!II II Iii" (ilil nlll ~70 . :tllI IIUt

U:,..., r,~:I.I' " ".::.1 ....,1 H UII~I ,; II

/1/1. ///1// 1/1111111/ III~:IlUI7 U,IM' 11.11 I' -, ;!·11I1I2 _llll:!h . HI ,. ('Ill :!I:!I.: 1'111"-1 ::;, II .I ~I:!'J.~ 1-1:.7tl 1.;1 L ~II'/II(~ ~ lJ tlI171 em H:B!"t ·1-1·,Ir! IIINIII It 5111:1 CIH:! I;,:'",::! HI·I;. 1111 II (HI. :!·I;'h" I' EUt,,,,:! 1111 E e -I:!,Ut:t·1 "'10111 ;!, II e crn~pup(l·l).0.~.;:!II:!:!I"I H I: ,. N ,?:! 111:'"•. 7 ·1.11":11 II:! I' .... :!.')'!,l.IJ IX'!:!:! .I 11;·1 I. IO

IO I:!;I·I4"h "';:;1:- .I ~I ,. !'1lti:i'~ ~,; :UIti~fI CII!:!;,;IIIII :1,,;.'1-1 .I ..H II .:!:',;I h ·lj·»IH ..5;. II (lll~ :1~1I14 CIII

I:!:'.·JI·I I;I-Ih .:111 llll: ~41141 cnl::,dh" .~I:I.lJ; 1 .11 " :!hhl :! :1'1:!:'"I; II I'" :!7Hh ~I :llIh',!" ", II E:':';",1 H l"h:!h UII:lj II e :':';'·'H ·1-1;';" III SII:!:II~tl +1;,ltl' ... II" .-: :!71Ultl ;,.lu:!1 II "' '"' :c 0 ;:111:\', I-U·I;, I~ II :.!Ill~.', ~1~;Il; :'::l " I' N ....

N :!1I·lnu .uu,:.!·' I II :!H:illh ·UhH'1 :U II , ,;lll~!'II2IU,:I.;' .1""lt .1 .~I,I II :J.I~" 1:111 ,:!'II:l ~ :;1 .. ;~I I 11:1 II

IHU~ crn:UNtll I :\,1"1" .. :111 II ;UI:I:! H ~,l1l;.!h I h:! I. -I :1U:lol I ;l''''loll :!11 II, I'll' ;11·11 H ·Ut"H.:! .. /" II

r,~. \illllllolll 1ll,l~r,lIn _II 1"'~llIIolIIL'", hUII/,1I11111 "I"'"'' r"~""111~ ,,,'h"'lIll'~ Iltl.;1I7~.1 :1·1."1·1 .1 1.11 II '" .12IH.1 ·1J1:!2.:! 11·1. II ;\:!:.!;U. :I1'h:.!h " IHII ;: II :I:h2,1 ."\·J:!:'17 II ;1]:\1)10 ;ltill~4' .:!II II

EXAMPLE-Sn _ ~411l2 1'A 286.3nn1 g(Vo ) - ---

TC !J.v 5.4xl07s-1 3A12 g(V)~ g2 'A A 2l

10-3J e gl 87th (10-8 s)(l 0-2 em 2 ) hv

W17107 W/em 2 - I

heA 2l 'ACJ 12

10 5 !J.V 0.4em-3

3 4

3

SATURATION OF AN EXCITATION TRANSITION

Example: Yb

~~,:J 01- --------------,.-' --.......,. 01-- ,,.-,---

10,7 10. 6 10'~ o 2 , 6 6 10 12 1L 16 (tll _I, (Jcm· 21 (dl t (kVcm"1

Ikpcndcm:c of YII ion} iel,' '"I I hc lascr pulsc cnci gy Ihu

from V. S. Lelokhov, Laser Pholoionjzalion Speclroscopy

}~:L , t-+

IONIZATION CROSS SECTION TABLES

RIS Data Sheets - E. 8. Saloman, Spectrochimica Acta 458,37 (1990), 468,319 (1991) and 478,517 (1992)

(Mostly calculated cross sections with the code of R. D. Cowan)

IONIZATION CROSS SECTIONS RULES OF THUMB

Dependence on photon energy

Generally high near threshold and then fall off to higher energy

Dependence on valence electron to be removed

s-electrons: low cross sections

p-electrons: higher cross sections

d-electrons: even higher cross sections

IONIZATION CROSS SECTIONS ENERGY DEPENDENCE

Hydrogen ns electrons: Cesium 6p electron: 7'.

PhOton tonerqy leV I

n ~ .2

.1$ - .. p

\ - \ - I \

10-17 \: W1

"'~ . I I· \

10- 18 : I \ :;:: I \,.

10 - '~ 001 0 0' -o....J,IL--:..L._~.lo-'--L-~':'"::

j... i E i :: -:l '" Ju ~ (; u ~

~ ~ j-=

..oJ~ ! l

___ ._._•. ~l~_ •• ~. I .. .J ...." l: I:.'

from J. A. Paisner and R. W. Solarz, from V. S. letokhov, Laser Photoionjzation - -- --------"Resonance-Pholoionizalion- Splrctrcrs-copy"- --SQeCff9SCopy-

;,.'1:

IONIZATION CROSS SECTIONS L, EDEPENDENCE (RIS Tables)

• s electron ~ p elecrron o d electron

.. • • •

,p .u o

•• o • 0

2 3 4 5 6 6E (eV)

SATURATION OF THE IONIZATION STEP

N2Resonance step is saturated: N

Rate equation for ionization of atoms:

dN dN j J I - = -- = -cr· (-)N') = -cr· (-)(BN)d t d t I hV - I hV h.'" j

Solution:

Rb 6p slale

,.. ~ .. . - - - . '.,:t '"

Note the degeneracy factor B! (trom-Letokhov.-~ --- Pholoionization Spectroscopy)

IONIZATION CROSS SECTION EXAMPLE - Sn

One-step excitation to 5p6s state (UV+UV scheme):

Esat =11n1J / n11n2

2O"j =6.1 X 10-19 Cln

Two-step excitation to 5p7p state (UV+VIS+VIS scheme):

Esat = 0.45mJ / mm 2

O"j = 7.2 X 10-18 cln2

li104 lilll 11111 ~70 al8 11111 4bU ~ lIUI

11,,:1 "",", "'/1 "I'" ''1 _,I :Iu:!:m L:III :"",'",IJH,. ~11'n 1:111:

(! U, !,111:1 ."'11 [ ~ < 4;1"1"" 1"1110 :.. ••lll'(!.'l

N ,.. '" n

;;; '" :'l·u·d ~I: allli~1I I"nl ,II lJ; :IHII4 ("II ,I j lJ; :l4(J41 cnl ..

eC

'"~

E ~ Sri'" .."

c

" ~ ~ ~

'", '" :'1' .I" .. :"':!Il co,lI ~ Iti,,~ I II'

-'--- ---1.- tI 1'111

ISOTOPE RATIO MEASUREMENTS WITH RIS: SOURCES OF BIAS

Laser mode structure

Observed with broadband lasers?

Atomic hyperfine structure and isotope shifts

W. H. King, Isotope Shifts in Atomic Spectra E. W. Otten, "Nuclear Radii and Moments of Unstable Isotopes RIS Data Sheets - E. 8. Saloman, Spectrochimica Acta

458,37 (1990), 468,319 (1991) and 478,517 (1992)

Role of m-Ievels

selection rules different populations different ionization cross sections

P. Lambropoulos and Y. Lyras, Phys Rev. A40,2199 (1989) W. M. Fairbank, Jr. et aI., Phys Rev. A40,2195 (1989)