Dispersed fluorescence studies of jet-cooled HCF and DCF: Vibrational

Structure of the X1A state

Haiyan Fan, Calvin Mukarakate, Mihaela Deselnicu,

Chong Tao, and Scott A. Reid*

Department of Chemistry, Marquette University,

Milwaukee, WI 53201

Objectives

Obtain dispersed fluorescence (DF) spectra in the Ã1A" ←X1A' system of HCF and DCF following excitation of pure bending transitions and combination bands.

Fit the experimental term energies of both species to an effective Hamiltonian (Dunham expansion).

Compare experimental parameters with ab initio calculations.

Search for perturbations involving low-lying triplet state, predicted to lie ~ 5200 cm-1 above the ground state.To date the vibrational structure has been probed only up to ~4200 cm-1 above the vibrationless level.

Literature review Chang and co-workers observed ã3A" perturbations in DF

spectra of HCCl and HCBr and estimated the Singlet-triplet energy gap (∆EST), which was consistent with theory.

For HCF, predicted values for ∆EST range from 4617-5537 cm-1 consistent with an experimental value of 5210±140 cm-1 derived from photoelectron spectra of HCF- by Lineberger and co-workers.

Jacox and Mulligan reported infrared spectra of HCF and DCF and reported values for v2 (bend) and v3 (CF stretch).

Suzuki and Hirota reported DF and Stimulated Emission Pumping (SEP) spectroscopy of HCF from 00 and identified resonance between 11 and 2131.

Lineberger and co-workers reported values for ν3 of HCF and DCF from photoelectron spectroscopy of the corresponding negative ions.

Methodology

HCF and DCF were generated by pulsed electrical discharge through a 1-2% mixture of CH2F2(obtained commercially) or CD3F(synthesized from CD3OD via a Literature procedure) in Ar.

The laser system is an etalon narrowed dye laser (Lambda-Physik Scanmate 2E) pumped by third harmonic of a Nd:YAG laser (Continuum NY-61 or Powerlite 7010).

DF spectra were acquired using a 0.3 m spectrograph with 600 or 1800 l/mm grating and gated intensified CCD detector.

Spectra were acquired in photon counting mode, over typically 5000 laser shots with 100 m slit width.

Results and Discussion

0 2000 4000 6000 8000 10000

0 2000 4000 6000 8000 10000

0 2000 4000 6000 8000 10000

11121

0

Red shift in cm-1

Pump 11021

0

Pump 240

22031

1

Pump 22031

0

241

This shows DF spectra for HCF obtained by exciting rQo bandhead in

The x-axis is the vibrational

energy in the X1A' state. Two features are observed for

each vibrational state, corresponding to transitions to ground state levels with

Ka = 0, 2. Excitation of various overtone

and combination bands allowed Franck-Condon access to the majority of X1A' levels.

10

10

10

20

40 2132,2 and

Effective spectroscopic Hamiltonian (Dunham Expansion) DCF

The term energies are well described by this model.

A total of 40 levels were included in the fit and yielded a standard deviation of 1.9 cm-1.

HCF This model poorly

reproduces the term energies. Fitting all levels yields standard deviation of 22 cm-1.

This is due to resonance between the set of levels 112n, 2n+131 and 2n+2.

Only 18 levels are included in the fit that yields a standard deviation of 4.5 cm-1 (omitting 112n, 2n+131).

0 0 0 01 2 3 1 1 2 2 3 3

2 2 20 0 01 11 2 22 3 33

0 0 0 01 2 12 1 3 13 2 3 23 1 2 3 123

, ,

G

x x x

x x x x

Parameter HCF DCF1 2666(6) 1999.2(16)2 1417(2) 1062.2(5)3 1194(5) 1203.9(17)x11 -26(3) -35.1(8)x22 -3.4(3) -1.33(8)x33 -5.4(27) -9.6(9)x12 -56.8(22) -29.56(26)x23 -26.8(15) -6.54(22)x13 -5.0(23) -2.4(13)x123 -------  -0.2(5)

X1A' vibrational term energies fit parameters (in cm-1)

Comparison of experiment and theory for the vibrational frequencies of DCF and HCF in X1A'

Method ω1 in cm-1 ω2 in cm-1 ω3 in cm-1

HCF

oB3LYP/aug-cc-pVTZ 2767 1437 1198

oExperiment (DF) 2710(6) 1461(2) 1213(5)

DCF

oB3LYP/aug-cc-pVTZ 2032 1078 1199

oExperiment (DF) 2032.7(18) 1080.9(5) 1213(19)

HCF

An effective Hamiltonian which incorporated diagonal terms and Fermi resonance matrix elements between 112n, 2n+131 was utilized.

A fit to 33 levels improved the standard deviation from ~22 cm-1 to ~11 cm-1.

Higher order terms will need to be included to fully describe the intra-polyad coupling.

Searching for the triplet state

5000 5200 5400 5600 5800 6000

Pump 11

021

0

Red shift in cm-1

Pump 240

Pump 22031

0 A complicated vibrational

structure is observed in HCF near the predicted position of the ã3A" origin.

We expect a significantly larger A constant for triplet perturbed levels, as the calculated A constant for the vibrationless level of ã3A" is ~23 cm-1.

All six levels in this region display an A constant consistent with assignment to X1A', i.e., the derived constants fall between ~14.5 and 16.5 cm-1.

Searching for the triplet state

5000 5200 5400 5600 5800 6000

Pump 11

021

0

Red shift in cm-1

Pump 240

Pump 22031

0

This observation is also true at higher energies, although the constants increase on average.

We thus have not identified perturbations involving the ã3A" state – this may be due to small spin-orbit matrix elements (~ 0.5 cm-1 in the Ã1A' state).

Conclusions DF spectra were recorded following excitation of the pure bending

levels and the combination states in the Ã1A" ←X1A' system of HCF and DCF.

This reveal rich detail concerning the vibrational structure of the X state up to ~ 10 000 cm-1.

An effective spectroscopic Hamiltonian (Dunham expansion) works well for DCF but poorly reproduces the experimental term energies of HCF, where the spectra are complicated by resonances among the set of levels 112n,2n+131 and 2n+2.

Density functional calculations of the ground state vibrational frequencies were performed; the results are in excellent agreement with the experimentally derived vibrational parameters.

The search for perturbations involving the low-lying triplet state has to this point been unsuccessful – due to the small matrix elements, rotationally resolved spectra will be needed.

Acknowledgements

Petroleum Research FundNational Science Foundation