High resolution FTIR and diode laser supersonic jet spectroscopy of the N = 2 HF stretching polyad in (HF)2 and (HFDF): Hydrogen bond switching and predissociation dynamics

Yabai He, Holger B. Müller, Martin Quack*, Martin A. Suhm

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

7 Citations (Scopus)

Abstract

We report Fourier transform infrared (FTIR) and high resolution diode laser spectra (∼ 1 MHz instrumental bandwidth) obtained in cooled absorption cells as well as in a supersonic jet expansion for the N = 2 polyad region of the HF-stretching vibrations of (HF)2, HFDF and DFHF. Three vibrational transitions have been observed for (HF)2 and two for both monodeuterated isotopomers. For (HF)2 we have identified and analysed the observed transitions of the polyad member 22 of the type ΔKa = 0 and ΔKa = ± 1 up to rotational sublevel Ka = 3. Band centers as well as rotational constants of all four Ka states have been determined. The tunneling splittings due to hydrogen bond switching for these four Ka states have been investigated, with the Ka = 0 up to Ka = 2 sublevels having tunneling symmetry Γvt = A+ for the lower tunneling states, and switching periods ranging from 158 ps for Ka = 0 to 1.35 ns for Ka = 2. A tunneling level inversion is found at Ka = 3, leading to a symmetry Γvt = B+ for the lower tunneling state of this Ka-sublevel. The vibrational assignment of the measured spectra of (HF)2 was established by comparison with the monodeuterated isotopomers HFDF and DFHF. For HFDF we have identified and analysed five subbands between 7600 cm-1 and 7730 cm-1. We have determined the spectroscopic constants of the rotational levels Ka = 0 and Ka = 1 for the vibrationally excited state and of the levels of Ka = 1 and Ka = 2 of the ground state, the latter from combination differences. From the measurements in a supersonic jet expansion we determined the predissociation line width of the N = 22, Ka = 1 to be about 120 MHz for the Γvt = A+ tunneling state of (HF)2 and about 90 MHz for Γvt = B+. For the Ka = 0 level of N = 22 we obtained predissociation line widths ranging around 100 MHz, similar to those of the Ka = 1 level. In the case of HFDF, the predissociation line width of Ka = 1 is about 80 MHz. Predissociation lifetimes for these levels with the unbonded HF stretching excited thus are in the range of about 1 to 2 ns. The predissociation width in the N = 21 level is uncertain by about a factor three with Ig(Δv/MHz) = (3 ± 0.5) and in N = 23 it is about 600 MHz corresponding to rounded lifetimes of 0.1 ns and 0.3 ns when the bonded HF stretching is excited thereby demonstrating strongly mode selective predissociation rates in the N = 2 polyad. Under thermal equilibrium conditions we derived the pressure broadening coefficient for (HF)2 (γ = (6 ± 1) × 10-4 cm-1/mbar in the wavenumber range between 7713 cm-1 and 7721 cm-1 for total gas pressures between 10 and 60 mbar, all values as full widths half maximum). For absolute frequency calibrations we have remeasured the first overtone transitions of the monomer HF with much improved precision between P(5) (7515.80151 cm-1) and R(7) (7966.22188 cm-1).

Original languageEnglish
Pages (from-to)1581-1645
Number of pages65
JournalZeitschrift fur Physikalische Chemie
Volume221
Issue number11-12
DOIs
Publication statusPublished - 2007

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