LASERLASER Induced Induced Fluorescence of IodineFluorescence of Iodine

Eðlisefnafræði 5 – 30. mars 2006Eðlisefnafræði 5 – 30. mars 2006

Ómar Freyr Sigurbjörnsson Ómar Freyr Sigurbjörnsson

Introduction

Iodine is the heaviest common halogen and exists as a solid at room temperature in sublimation equilibrium with its vapor.

Its vapor has the appearance of a violet gas, indicating a visible absorption. This absorption corresponds to a spin-forbidden transition from the lowest vibrational levels of the singlet electronic ground state to high vibrational levels of a triplet excited state.

B 3u X 1g

By laser excitation, the reverse process of fluorescence is studied

The experimental setup

Experimental

Iodine crystals were inserted inside the fluorescence cell and the cell was then vacuated and heated

The monochromator was set to a wavelength of 630 nm and slidt width of 2000 m for the indirect absorption measurement by scanning the Dye-Laser and recording the amount of fluorescence

The fluorescence spectra was then recorded by scanning the monochromator 1 nm/min and slit width of 500 m for a fixed Laser excitation wavelength

Getting a strong signal was difficult and the slit width could not be smaller, resulting in lower resolution

Optimally the scanning speed should be slower

Iodine absorption spectrum

Generally an absorption spectrum can mostly give us information on the physical characteristics of excited states of molecules

Alternatively, emission spectra contain information on the ground state

Measuring directly the absorption spectrum of a molecule using Laser light is impractical The light is so strong that almost no difference is detectable due to

molecular absorption So the absorption spectrum of Iodine is measured indirectly by

measuring the amount of fluorescence as a function of laser wavelength

Recorded Iodine fluorescence as a function of Laser wavenumber

120

100

80

60

40

20

Rel

ativ

e I

nte

nsity

1704017030170201701017000cm

-1

Rotational peak (17010.2 cm-1

) selected for Laser excitation

588,2 586,5 nm

Iodine absorption spectrum

Rotational structure is observed due to the vibrational transition v’=16 v’’=2 (B X) as determined by the UV/VIS absorption spectrum recorded and assigned in an experiment performed in Physical Chemistry 2

Laser wavelength of 17010,3 cm-1 (587,88 nm) is selected to excite iodine molecules to the specific rotational and vibrational state, then the fluorescence to the ground state vibrational levels is recorded

Iodine fluorescence spectra

The expected Iodine fluorescence spectra should have peaks corresponding to different vibrational levels of the ground state

Also, each vibrational peak should be rotationally split in two due to the fact that for each transition, the selection rule J= ± 1 applies

Recorded Iodine fluorescence spectra

633 nm 593 nm

160

140

120

100

80

60

40

20

0

Rel

ativ

e in

tens

ity

169001680016700166001650016400163001620016100160001590015800 [cm

-1]

v´´= 3

v´´= 4

v´´= 5v´´= 6

v´´= 7

Analysis of measured and calculated peak positions

A transition between two electronic states is described by the equation (neglecting rotational contribution)

(v",v’ ) = el + G(v’) - G(v")

v’’+½ measured [nm]

measured [cm-1]

calculated [cm-1]

3,5 593,5 16850 16892

4,5 600,7 16645 16682

5,5 608,7 16429 16474

6,5 615,8 16238 16267

7,5 623,9 16028 16061

Where G(v) = e(v+ ½) - ee (v + ½)2

By plotting measured against v’’+½ the constants e and ee can be determined

16800

16700

16600

16500

16400

16300

16200

16100

[c

m-1

]

7654V''+1/2

y = 1,0714x2

- 216,89x + 17596

measured [cm-1]

Huber & Herzberg [cm-1]

e216,9 214,50

ee1,07 0,614

Results

Discussion The rotational split mentioned before can be seen in the recorded

spectrum shown earlier If the data and resolution are good enough, analysis can be

performed to assign the rotational quantum number J’ and rotational constants B for the ground vibrational states

The following equation describes the relationship E(J’,V’’) = 2(2J’+1)Be’’ - e’’ 2(2J’+1)(v’’+ ½) By plotting E as a function of v’’+ ½ and using known values for

Be’’ & e’’, the constant J’+1 can be found and consequently other B values

Unfortunately my recorded spectrum does not yield satisfactory results by such treatment

Discussion

There are a few things that could explain this Problems with signal strength and noise Scan speed and slit width to high Some data points from laser pulses are lost/do

not register on the computer so the data aquisition is not completly continous

More peaks and more scans are required for a thorough treatment

I wish to thank Mr. Wang for his help in performing this experiment