One- and Two-Color Laser Spectroscopy with Photoacoustic and Multiphoton Ionization Detection

The techniques of pulsed photoacoustic spectroscopy and multiphoton ionization spectroscopy have been used to study several types of weak optical transitions in the ultraviolet or visible spectral regions for gas phase molecules. Both one- and two-color experiments have been performed. The one-color experiments involved the study of high vibrational overtones and spin-forbidden transitions. The two-color experiments demonstrated the application of photoacoustic detection of stimulated emission pumping for populating high vibration levels of the ground electronic state. Two-color experiments were also used to study excited state absorption processes in aniline. Pulsed laser photoacoustic spectra of CD3H in the region of the ∇vCH = 5, 6, and 7 CH stretch overtones were obtained. These overtones and nearby combination bands displayed sharp rotational structure, indicating a minimum excited state population relaxation time of 5 x 10-12 sec. A combination sum analysis was used to generate excited state rotational constants B'. These constants reflect the geometries of the excited states, and therefore provide a sensitive probe of the mixing between various zero order excited states. Photoacoustic detection was used to monitor a stimulated emission pumping process in p-difluorobenzene. Using the Ã1B2u 5' state as an intermediate state, several vibrational levels of the ground electronic state were populated. The photoacoustic technique is an attractive alternative to other techniques because of its sensitivity, simplicity, and its ability to differentiate between stimulated emission pumping and excited state absorption. Time-resolved excited state absorption studies were performed on aniline using the multiphoton ionization and photoacoustic detection techniques. Signals from both of these techniques were measured as a function of the time delay between two laser pulses of different wavelength. The first pulse excited the S1 0-0 transition. The second pulse excited either S1 or a triplet state produced by intersystem crossing, depending on the time delay between pulses. Both ionization and dissociation processes were observed. By varying the conditions of excitation, it appears that a given amount of energy can be selectively channeled into either ionization or dissociation pathways. These results were explained using a simple Franck Condon factor model. The application of pulsed laser photoacoustic spectroscopy to the study of weak absorptions in the visible and ultraviolet spectral regions was investigated. Photoacoustic spectra of triplet states in CS2, SO2, biacetyl, and thiophosgene were obtained. An attempt to detect the ã3B1u state in benzene, produced instead a two-photon spectrum of several Rydberg states. It was discovered that the sensitivity of the pulsed photoacoustic technique for extremely weak absorptions is limited by the increased probability of multiphoton absorption when high intensities are used. Finally, in two appendices, the detection of the ã3A2 state in CS2 by the multiphoton ionization technique is presented. The CS2 study demonstrates the utility of the multiphoton ionization technique for detecting spin-forbidden transitions at high resulution and with great sensitivity.