Vibrationally mediated photodissociation of t-butyl hydroperoxide: Vibrational overtone spectroscopy and photodissociation dynamics

Abstract

Vibrationally mediated photodissociation is a two-photon technique for studying the spectroscopy and photodissociation dynamics of highly vibrationally excited molecules. In these experiments, a highly vibrationally excited t-butyl hydroperoxide (t-BuOOH) molecule, prepared by excitation in the region of the third overtone of the O–H stretching vibration (4νOH), absorbs a second photon to dissociate to OH and t-butoxy fragments, and laser induced fluorescence determines the quantum state populations of the OH fragment. Vibrational overtone excitation spectra, obtained by varying the vibrational overtone excitation wavelength while monitoring a single OH rotational state, are nearly identical to photoacoustic spectra. We fit the coarse structure in the vibrational overtone excitation spectrum in the region of the 4νOH transition and the photoacoustic spectra in the regions of the 5νOH and 6νOH transitions using a spectroscopic model of the interaction of the O–H bond stretching vibration with the torsional vibration about the O–O bond. This analysis determines the barrier to internal rotation of the O–H and t-butoxy groups through the trans configuration and its variation with vibrational excitation. The trans barrier in the ground vibrational state is 275 cm−1 and increases with vibrational excitation to 425, 575, and 680 cm−1 for t-BuOOH molecules with four, five, and six quanta of O–H stretching excitation, respectively. Comparison of the energy disposal in the vibrationally mediated photodissociation with that for direct photolysis at 376 nm, which adds the same amount of energy to the molecule, illustrates the unique dynamics that can occur when vibrational excitation precedes photodissociation. Single-photon photolysis produces fragments with large recoil velocities, while vibrationally mediated photodissociation produces slowly recoiling fragments having substantially more energy in internal excitation.

Controlling bond cleavage and probing intramolecular dynamics via photodissociation of rovibrationally excited molecules

“…An additional point should be raised concerning the agreement between the measured (OD ‡ H)/(OH ‡ D) branching ratios and the quantum scattering calculations [47,48]. In the VMP studies of 4¸O H and 5¸O H [55,58] and in the 157 nm photolysis of ground state HOD [59] the agreement was very good.…”

“…In another set of experiments, molecules containing substantial vibrational energy, four to six quanta of O-H stretch, including hydrogen peroxide (HOOH) [46], nitric acid (HONO 2 ) [47], and tert-butyl hydroperoxide ((CH 3 ) 3 COOH) [48], were photodissociated . In these experiments the O-H excitation energy was intramolecularly coupled into the O-O or O-N stretch that became the cleaved bond and they particularly emphasized the comparison of product state distribution from VMP to those from an isoenergetic single photon photolysis.…”

Level distributions, partition functions, and rates of chirality changing processes for the torsional mode around O–O bonds

“…The effective kinetic energy operator that we have employed for the calculation of energy levels is of the type used for example by Likar et al 22 for the HOOR systems. We have applied it for general ROORЈ systems, where R and RЈ can be an atom or a group of atoms.…”

Photodissociation of rovibrationally excited C2H2: Observation of two pathways

“…The vibrational excitation prior to photodissociation greatly enhances the C-H bond cleavage. One of the lines, R (13), in the REMPI action spectrum is standing out of the rotational contour and also shows a wider H profile. The observed intensities and linewidths suggest two photodissociation pathways following the preparation of rovibrational states of C 2 H 2 , where the C 2 H fragment is produced in two different states.…”