Infrared–ultraviolet double resonance studies of benzene molecules in a supersonic beam

Abstract

We have used IR excitation to selectively create populations in admixtures of the zeroth-order states comprising the ∼3000 cm−1 ‘‘C–H stretching Fermi triad’’ of benzene. UV spectra of the 260 nm Ã(1B2u)←X̃(1A1g) transition in the IR-excited molecules show several new bands, which we have assigned. Final states in the UV transitions are some vibrational levels which have not been detected before, allowing us to find several excited-state vibrational frequencies. We have determined ν′3 =1327±3 cm−1, ν19 =1405±3 cm−1, and ν′20 =3084±5 cm−1. Also, vibrational structure which was unresolved in IR spectra of the ‘‘Fermi triad’’ was resolved in the UV double resonance spectra, confirming that the C–H stretching admixture is really a tetrad. The 3048, 3079, and 3101 cm−1 states had formerly been given the labels ν″20, ν″8+ν″19, and ν″1+ν″6+ν″19, respectively. Actually, the middle level most nearly resembles ν″1+ν″6+ν″19, and the 3101 cm−1 level is strongly mixed with ν″3+ν″6+ν″15. As predicted by molecular orbital theory, excited-state C–H bending and stretching frequencies are not very different from those in the ground state. Furthermore, we suggest that the four C–H stretching frequencies increase uniformly by ∼20 cm−1 in the excited state; reexamination of the Atkinson and Parmenter 260 nm Ã←X̃ spectrum leads us to reassign ν2 from 3130 to ∼3093 cm−1, which is 19 cm−1 above ν″2. There is a Fermi resonance between the ν6+ν′20 level and another level ∼13 cm−1 lower in energy; the strength of the perturbation is ∼18 cm−1. Possibilities for the perturbing vibrational state are ν6+ν′8+ν14 and ν′6+ν13.

Mid-IRspectra of different conformers of phenylalanine in the gas phase

“…Ionization of the molecules is performed via a resonant 1-color 2-photon process (R2PI) around 260 nm and IR spectra are obtained via IR ion dip spectroscopy 4,[19][20][21] The UV-excitation laser is then parked on the S 1 -S 0 transition of the conformer of interest, and the IR laser, which is fired in time and position just before the UV laser, is scanned over the fingerprint region of the molecule (300-1900 cm…”

Remarkable Site Difference of Vibrational Energy Relaxation in Benzene Dimer: Picosecond Time‐Resolved IR–UV Pump–Probe Spectroscopy

“…The S 1 -S 0 transition from the level at 3077 cm À1 occurs mostly by the Dv 1 = À1 and Dv 6 = À1 transitions, because the main character of this level is n 1 + n 6 + n 19 . [4] The h*(Stem) molecule exhibits a strong vibronic band, while h*(Top) shows a weak progression on the higher frequency side of the band of h*(Stem).…”

Molecular and supramolecular chirality: R2PI spectroscopy as a tool for the gas‐phase recognition of chiral systems of biological interest

“…Developed by Lee and coworkers, 36,37 this method was first used by Riehn et al 38 to investigate intracluster ionmolecule reactions following R2PI. Most of the experiments have been performed employing IR laser systems in the spectral region where the CÀ ÀH, OÀ ÀH, NÀ ÀH, and C¼ ¼O stretching fundamentals are found, but also exten- sion to the mid-IR (fingerprint) spectral region has been achieved by the coupling of R2PI to Free Electron Laser (FEL) radiation.…”

“…By measuring the ion signal while tuning the IR wavelength, an ion-dip spectrum is recorded. [36][37][38][39][40] The so-called IR depletion spectrum represents the ground state vibrational spectrum of the probed species. It allows the discrimination of different conformers and to elucidate the size and structure of the ground state species.…”