An angle resolved dissociative photoionization study of the c4 state in O using the TPEPICO technique

A Padmanabhan1, M A MacDonald2, C H Ryan2, L Zuin2 and T J Reddish1

1 Physics Department, University of Windsor, Windsor, ON, Canada

2 Canadian Light Source, University of Saskatchewan, Saskatoon, SK, Canada

†Email: reddish@uwindsor.ca Web-Site: http://www.uwindsor.ca/reddish

u

2

The vibrational levels = 0, 1 produced from dissociative photoionization of O c4 state at ~24.56 eV have distinctly different lifetimes, , which diminish their

inherent anisotropic photoion angular distribution characterized by a parameter. The = 1 level decays to the L2 limit by tunneling through the potential barrier. Pulse-field ionization photoelectron (PFI-PE) experiments [1] determined the lifetime for = 1 as 6.9 ± 0.7 x10-14 s and this has been recently supported by theoretical studies [2,3]. In contrast, the = 0 level lives long enough to fluoresce to the b4 state [4,5] and dissociative ionization competes with radiative decay. Akahori et al [6] also find a weak L5 contribution (~5%) after subtracting L5 yield due to the underlying underlying continuum, a background contribution that is also observed by [5,7,8]. Richard-Viard et al [5] also quantify the O+/O2

+ ratio as 6 ± 1 for the = 0 level; i.e. a ~15% fluorescence

branching ratio.

Vibrational Level

Dissociation Products Limits Dissociation Energy (eV)

= 0 O 3P + O+ 4S(spin-orbit coupling)

L1 18.733

O 1D + O+ 4S(tunneling)

L2 20.700

O 3P + O+ 2P(continuum)

L5 23.750

= 1 O 1D + O+ 4S(tunneling)

L2 20.700

Introduction

[1] Evans et al 1998 J. Chem. Phys. 109 1285[2] Hikosaka et al 2003 J. Phys. B . 36 4311[3] Demekhin et al 2007 Rus. J. Phys. Chem. B. 2 213[4] LeBlanc 1963 J. Chem. Phys. 38 487[5] Richard-Viard et al 1987 J. Phys. B . 20 2247[6] Akahori et al 1985 J. Phys. B . 18 2219 [7] Frasinski et al 1985 J. Phys. B . 18 L129 [8] Ellis et al 1994 J. Phys. B. 27 3415[9] Lin and Lucchese 2002 J. Chem. Phys. 116 8863

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Toroidal SpectrometerA Threshold Photoelectron-Photoion-Coincidence (TPEPICO) experiment was performed using a dual toroidal spectrometer [10] in conjunction with linearly polarized synchrotron radiation on the VLS-PGM 11ID-2 beamline at the Canadian Light Source (CLS) [11]. The spectrometer utilizes toroidal analyzers, which have properties ideally suited for measuring charged particle angular distributions since they energy select the photoions while preserving the initial angle of emission. The threshold electrons were obtained using the penetrating field technique [12], which is explained in Fig 3, in conjunction with the smaller toroidal analyzer shown in Fig 2.

Fig 2: A schematic diagram of the mutual configuration of the two toroidal analyzers in our detection geometry. The photon beam is perpendicular to the page and the polarization direction is horizontal. The TPEPICO signal corresponds to threshold electrons yield (over 4 sr) measured in coincidence with energy-resolved ions with emission angles within the central ~160 grey sector of the (180) toroidal analyzer. The out-of-plane emission angular acceptance in the ion channel is ~ ±10°.

[10] Reddish et al 1997 Rev. Sci. Instrum. 68 2685[11] Hu et al 2007 Rev. Sci. Instrum. 78 08109[12] Cvejanovic and Read  1974 J. Phys. B . 7 1180

Fig 3: Trajectories of 0.001eV electrons emitted over 4 sr from a point source can be focused and collimated by the weak electric field from an ‘extractor’ electrode that penetrates through the zero volt aperture. The solid angle of extracted, faster electrons is significantly smaller than for these "threshold" electrons and rapidly diminishes with electron energy. This highly-efficient, energy selective extraction allows 'threshold electron spectroscopy’.

Gas needle

ThresholdElectronDetector

ToroidalPhotoionDetector

[15] Sadeghpour et al 2000 J. Phys. B . 33 R93[16] King and Avaldi 2000 J. Phys. B . 33 R215[17] Bouri et al 2007 J. Phys. B . 40 F51[18] Cvejanovic et al 1995 J. Phys. B . 28 L707[19] Wehlitz et al 1999 J. Phys. B . 32 L635 [20] Thompson et al 1998 J. Phys. B . 31 2225

Fig 6: Threshold photoelectron spectrum (TPES) of Helium. Insert: The characteristic “cusp” [16,18] at the double ionization threshold energy of 79.0 eV.

Threshold Photoelectron Spectrum of HeliumThe double ionization region in helium continues to be the subject of intense interest [15-20], since it is the archetypal electron correlation system. Fig 6 shows the well-studied threshold region displaying the characteristic cusp at 79 eV, obtained with a resolution of 10 meV. The ratio of the threshold yield immediately below and above 79 eV is 1.08, in good agreement with our earlier study [18] obtained with ~ 70 meV resolution and still at variance with the 1.25 value from [20]. 77.8 78 78.2 78.4 78.6 78.8 79 79.2 79.4

0

1 103

2 103

Helium

Photon Energy (eV)

Experimental Outline We have investigated the angular distributions of ~2 eV O+(4S) ions following dissociative

photoionization from the c4 state for both = 0 and 1 using the threshold photoelectron-photoion

coincidence (TPEPICO) technique. Due to axial recoil in a diatomic molecule, the ion energy is simply given by: = (h - D) / 2 , where D is the dissociation limit. The threshold photoelectron

yield peaks at h = 24.564 and 24.756 eV for = 0 and 1 levels, respectively, the corresponding

values of are 1.932 and 2.028 eV for the dissociation limit of 20.7 eV. The toroidal analyzer used

to detect ions was operated with an energy resolution of E = 0.5 eV, which is much broader than

the ~100 meV spacing when set to detect 2.0 eV ions, and can readily separate ions from the

neighbouring dissociation limits at 18.733 and 22.057 eV.

u

Fig 4(b): A zoomed region of the c4 state, showing the = 0 , 1 levels at 24.564 and 24.756 eV, respectively. As in other photoelectron studies [8,13,14], we also find a very weak broad feature corresponding to = 2 at 24.97 eV on the sloping background of the 2 continuum. The energy resolution is 3.5 meV (FWHM) using He+

(n = 1).

20 20.5 21 21.5 22 22.5 23 23.5 24 24.5 250

2 103

4 103

6 103

O2+

O(3P) + O+(2D)

O(1S) + O+(4S)

O(3P) + O+(2D)

x6

Photon Energy (eV)

O(1D) + O+(4S)

Threshold photoelectron spectrum for O2+

Fig 4(a): The threshold photoelectron spectrum (TPES) for O2+ between 20-25 eV.

The dissociative ionization limits are indicated, as are the two most intense vibrational series: B2   and c4 .

[13] Guyon and Nenner 1980 App. Opt. 19 4068-79, [14] Baltzer et al 1992 Phys Rev A 45 4374

[21] Tanaka and Yoshimine 1979 J. Chem. Phys. 70 1626[22] Liebel et al 2002 J. Phys. B. 35 895 [23] Ehresmann et al 2004 J. Phys. B. 37 4405 [24] Demekhin et al 2007 Rus. J. Phys. Chem. B. 2 213[25] Lafosse et al 2002 J. Chem. Phys. 117 8368-84

Results and Analysis

Results of energy width analysis from Fig 4(b) and angular distribution fitting from Fig 5 in comparison with results of previous studies.

bSingle and double excitation configuration interaction (SDCI).

C 1.1 meV is their upper limit from experimental observation, corresponding to a lower limit on 0 ; 0.05 is an estimate from the

model presented in [2] .

Summary• Lower limit on 0 ~1 x10-12 s, corresponding to a width of < ~1 meV.

• 1 = 6.0 ± 0.3 x10-14 s in agreement with previous measurements.

• = 0.40 ± 0.05, is significantly smaller than predicted [9], namely

≥ 1.6, but in good agreement with observations by Lafosse et al [25].

values are 0.10 ± 0.02 and 0.30 ± 0.04 for = 0 and = 1 respectively ;

in good agreement with ~ 0 and 0.35 observed in [25] using electron-ion

vector correlation techniques.

• Our estimate of the energy width of 120 ± 20 meV for the = 2 level,

corresponding to 2 = 5.5 x10-15, is in excellent agreement with the results of

recent calculations [22,23,24].

T

O

mO

Funding Agencies:

T

O

mO

(eV) 24.564 24.756 25.005

Theory / Exp (meV) (s) (meV) (s) (meV) (s)

[21] T 6.6 x10-5 10 x10-9 0.013 5 x10-11 1.6 4 x10-13

[21] T (SDCI)b 0.019 3.5 x10-11 3.6 1.8 x10-13

[1] E 2.4 2.7(3) x10-13 9.5 6.9(7) x10-14

[22] T 0.19 3.4 x10-12 10.4 6.3 x10-14 167 3.9 x10-15

[2] Ec < 1.1 > 6 x10-13 9.5 6.9 x10-14

[2] Tc 0.05 1.3 x10-11 9.5 6.9 x10-14

[23] T 0.056 1.17 x10-11 13.2 4.99 x10-14 112 5.88 x10-15

[24] T 0.054 1.22 x10-11 9.7 6.8 x10-14 142 4.6 x10-15

This Work < 1 > 1 x10-12 11.0 ± 0.5 6.0 ± 0.3 x10-14 120 ± 20 5.5 ± 1.0 x10-15

c4 u

h

0 1 2

Fig 1: Potential Energy Curve [9] showing the dissociation limits for O2

+

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EO

E O

24.4 24.6 24.8 25 25.20

1000

2000

3000

4000

24.56 24.59 24.620

15000

30000

He

x30

Photon Energy (eV)

Angular Distributions

Fig 5(a): ‘true’ coincidences corresponds explicitly to the angular distribution ratio of = 1 to = 0 TPEPICO yield; 5(b): ‘random’ coincidences (i.e. completely uncorrelated in time) at h = 24.756 and 24.564 eV. The measured black data points between 180° and 270° have been reflected in the x and y axes to give the grey points.

(a) (b)

TO

These ratios can be found by assuming that the natural asymmetry parameter ,for a non-rotating

molecule is related to the measured value via with , where

is the rotational velocity of the molecular state and τ is its lifetime [25]. The dashed blue curve is

the predicted ratio using = 1.6, 0 = 1.2 x10-11 s and 1 = 6.0 x10-14 s , arbitrarily normalized to the

measured data. Using these values, the solid red curve is fitted to the measured data resulting in a

lower value for = 0.40 0.05 . The corresponding values are 0.10 0.02 and 0.30 0.04 for

= 0 and = 1, respectively, and are in good agreement with 0 and 0.35 observed in [25]. m

O

TO

TO

/

2 2 4 1 a a

T

O

m

O

TO m

O

cos1

cos1

200

211

P

P

OO

OO

In the Fig 5, the angular distribution ratios are proportional to: for (a) and

for (b); the factor 2.1 comes from the ( = 0 / = 1) threshold yield.

/1a

cos1

cos11.2cos1

200

20

211

P

PP

OO

OOO

0

30

60

90

120

150

180

210

240

270

300

330

0

30

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