Local modes of benzene and benzene dimer, studied by infrared–ultraviolet double resonance in a supersonic beam

Abstract

We used rotational cooling of molecules to -5°K by supersonic expansion and state-selective, multilevel saturation spectroscopy to obtain high-resolution spectra of the fundamental and first and second overtone transitions of C-H stretching modes in benzene and its dimer.Greatly reduced linewidths (<3 cm FWHM) in the rich spectra show that previously reported spectra have suffered from inhomogeneous congestion. Our observed spectral widths indicate that the vibrational lifetimes of the C-H stretches are at least a few psec, even at the energy of the second overtone (8800 cm 1 .) The "local mode" picture appears to apply when at least 3 quanta of C-H stretching motion are present. Spectra of the dimer are similar to those of the monomer but show a red shift of a few cm 1 , the appearance of combination bands involving van der Waals vibrational modes, some intensity changes, and a broadening of spectral features that increases with the vibrational energy. The dimer's predissociation lifetime at -3000 cm 1 vibrational energy exceeds -3 psec.

Vibrational overtone spectroscopy of pyridine and related compounds

“…The progressions of the two peaks for each compound were assigned to the two types of inequivalent CH oscillators. The 2,5 and 3,4-dibromothiophene spectra corroborate these assignments since each of these spectra possess only one absorption feature at 4 vCH. The member of the higher energy progression appears for 3,4-dibromothiophene and that of the lower energy progression appears for the 2,5-dibromothiophene.…”

“…These spectra are displayed to highlight the comparisons between them. The fundamental CH stretch region is included for completeness although these spectra have previously been reported13,1 4 . Fitting these transition energies to BirgeSponer plots, we find the mechanical frequencies and anharmonicities listed in Table 2.…”

“…Theoretical work 3 modeled the widths of these overtone absorptions using intramolecular vibrational relaxation pathways. Experimental work 4 on the second overtone spectrum of jet cooled benzene indicates homogeneous linewidths as small as 4 cm-I. The absorption spectrum of jet cooled benzene displays as many as seven absorptions in the second overtone region 5 .…”

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

“…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.…”

Mass selective infrared spectroscopy using a free electron laser

“…In this way, absorption of only a few photons can be detected, which is clearly impossible to be detected in direct absorption measurements. Some multi-photon fluorescence or ionization studies where one photon is in the IR do exist [4][5][6][7][8]. In addition, Esherick et al showed greatly improved sensitivity in stimulated Raman spectroscopy by combining this technique with resonant laser ionization [9,10].…”