Dispersed fluorescence studies of jet-cooled HCF and DCF: Vibrational Structure of the X 1 A state

Objectives Obtain dispersed fluorescence (DF) spectra in the à 1 A " ←X 1 A ' 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 ã 3 A" perturbations in DF spectra of HCCl and HCBr and estimated the Singlet- triplet energy gap (∆E ST ), which was consistent with theory. For HCF, predicted values for ∆E ST range from 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 v 2 (bend) and v 3 (CF stretch). Suzuki and Hirota reported DF and Stimulated Emission Pumping (SEP) spectroscopy of HCF from 0 0 and identified resonance between 1 1 and Lineberger and co-workers reported values for ν 3 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 CH 2 F 2 (obtained commercially) or CD 3 F(synthesized from CD 3 OD 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 This shows DF spectra for HCF obtained by exciting r Q o bandhead in The x-axis is the vibrational energy in the X 1 A ' state. Two features are observed for each vibrational state, corresponding to transitions to ground state levels with K a  = 0, 2. Excitation of various overtone and combination bands allowed Franck-Condon access to the majority of X 1 A ' levels.

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

ParameterHCFDCF 11 2666(6)1999.2(16) 22 1417(2)1062.2(5) 33 1194(5)1203.9(17) x (3)-35.1(8) x (3)-1.33(8) x (27)-9.6(9) x (22)-29.56(26) x (15)-6.54(22) x (23)-2.4(13) x (5) X 1 A' vibrational term energies fit parameters (in cm -1 )

Comparison of experiment and theory for the vibrational frequencies of DCF and HCF in X 1 A' Methodω 1 in cm -1 ω 2 in cm -1 ω 3 in cm -1 HCF o B3LYP/aug-cc-pVTZ o Experiment (DF) 2710(6)1461(2)1213(5) DCF o B3LYP/aug-cc-pVTZ o Experiment (DF) (18)1080.9(5)1213(19)

HCF An effective Hamiltonian which incorporated diagonal terms and Fermi resonance matrix elements between n, 2 n 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 A complicated vibrational structure is observed in HCF near the predicted position of the ã 3 A " origin. We expect a significantly larger A constant for triplet perturbed levels, as the calculated A constant for the vibrationless level of ã 3 A " is ~23 cm -1. All six levels in this region display an A constant consistent with assignment to X 1 A ', i.e., the derived constants fall between ~14.5 and 16.5 cm -1.

Searching for the triplet state This observation is also true at higher energies, although the constants increase on average. We thus have not identified perturbations involving the ã 3 A " state – this may be due to small spin-orbit matrix elements (~ 0.5 cm -1 in the à 1 A ' state).

Conclusions DF spectra were recorded following excitation of the pure bending levels and the combination states in the à 1 A " ←X 1 A ' system of HCF and DCF. This reveal rich detail concerning the vibrational structure of the X state up to ~ 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 n,2 n and 2 n+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 Fund National Science Foundation

Methodω 1 in cm -1 ω 2 in cm -1 ω 3 in cm -1 Reference HCF HF (scaled by 0.9) CCSD(T)/cc-pVQZ QCISD/6-311+G(d,p) (scaled by 0.954) QCISD/6-311+G(2d,p) QCISD(T)/6-311+G(d,p) CCSD(T)/6-311+G(2d,p) B3LYP/6-311+G(2d,p) B3LYP/aug-cc-pVTZ This work Experiment (DF)2710(6)1461(2)1213(5)This work DCF B3LYP/aug-cc-pVTZ This work Experiment (DF)2032.7(18)1080.9(5)1213(19)This work

Methodology HCF and DCF were generated by pulsed electrical discharge through a 1-2% mixture of CH 2 F 2 (obtained commercially) or CD 3 F(synthesized from CD 3 OD 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 lines/mm grating and gated intensified CCD detector. Spectra were acquired in photon counting mode, over typically 5000 laser shots with 100  m slit width. Calibration was done using a Fe:Ne hollow cathode lamp. Background spectra were obtained with the laser blocked to check for emission lines from species in the discharge.

DF spectrum of DCF

Excitation of various overtone and combination bands allowed Franck-Condon access to the majority of X 1 A ' levels. All four X 1 A ' levels in this region are observed; note:3 2 is only observed in the DF spectrum from For DCF we observe transitions to 19 of the 21 levels lying below ~ 4500 cm -1. DF spectra of HCF

We obtained limited rotational information from the observed K a " = 0,2 spacing, which equals 4 (A-B). The results show the expected increase in (A-B) with bending excitation. Rotational Constants of HCF

To compare this data to theory harmonic frequencies were deduced using the relationship: The fundamental frequencies were v 1 (bend) (16), v 2 (CF-stretch) (17) and v 3 (CD-stretch) (16).

Effective Hamiltonian