L. J. Rothberg

Multiphoton ionization of metal atoms produced in the photodissociation of group VI hexacarbonyls

Abstract The photodissociation of M(CO) 6 , M = Cr or W, and the multiphoton ionization spectra of the resulting atoms M(I) have been studied using a tunable visible dye laser M(I) fragments are produced throughout the range between 363 and 585 nm, and multiphoton absorption allows all of the states formed to be probed.

Electronic spectra of butadiene and its methyl derivatives: A multiphoton ionization study

The multiphoton ionization (MPI) spectra of trans‐1,3‐butadiene and three of its methyl substituted derivatives have been recorded using a tunable visible dye laser. The effect of methyl substitution on the electronic structure of butadiene is investigated, resulting in a reassignment of the spectra of these molecules. Quantum defects are found to be independent of principal quantum number, indicating little mixing between Rydberg and core orbitals. The validity of retaining conventional s, p, d, and f descriptions of Rydberg orbitals in these quasilinear molecules is demonstrated. Two electronic states of butadiene, assigned to the 3d and 4s Rydberg states, are observed for the first time in optical spectroscopy. No evidence for an excited 1Ag valence state is found in the MPI spectra.

Gas phase multiphoton photodissociation of Mn2(CO)10: The effect of collisions on photofragmentation

Multiphoton dissociation of Mn2(CO)10 followed by multiphoton ionization of the generated fragments is accomplished with a single tunable pulsed dye laser. Efficient dissociation to bare Mn atoms occurs over a broad range of wavelengths via a sequential photodissociation mechanism. Spectral changes upon addition of buffer gases are shown to reflect collisional effects on intermediates in the dissociation process.

Time resolved photoacoustic spectroscopy applied to properties of picosecond transients

Time resolved photoacoustic spectroscopy represents a promising new approach to the study of time resolved absorption with picosecond time resolution. The sensitivity of photoacoustic detection permits investigation of dilute and fluorescent samples which are difficult to study by conventional flash photolysis. Theory shows that quantitative measurements of important transient properties and new insights into photochemical and photophysical dynamics can be obtained by measuring heat deposition as a function of probe fluence. Experimental determinations of absorption and stimulated emission cross sections, and lower limits for rates of nonradiative relaxation of highly excited states, are presented.

Evidence of a state dependent depletion process in the two‐photon fluorescence excitation spectra of saturated amines

The two‐photon fluorescence excitation (TPFE) spectra of regions of the ? states of two saturated amines 1‐azabicyclo [2.2.2]octane (ABCO) and trimethylamine (TMA) are reported. These spectra are compared with the respective one‐photon absorption (OPA), one‐photon fluorescence excitation (OPFE), and multiphoton ionization (MPI) spectra for both molecules. For ABCO, this comparison clearly indicates major differences in both the vibronic band intensities and the amount of sequence structure present in the TPFE spectrum relative to the MPI, OPA, and OPFE spectra, which are all comparable. The ’’distortions’’ of the TPFE spectrum are interpreted in terms of a laser‐induced ? state‐dependent depletion process from ? which results in ionization. Pressure effects on the TPFE spectrum imply that the rate of this up‐pumping process depends strongly upon the particular vibrational modes excited in the two‐photon‐induced ?←? transition. A further implication of this interpretation is that the intramolecular vibrati...

Time-resolved photoacoustic spectroscopy in the picosecond regime

Abstract Photoacoustic detection has been applied, for the first time, to detect and study excited molecules and reactive intermediates on the picosecond timescale. Dilute and weakly-absorbing transients are readily observed under conditions which render them inaccessible to conventional methods.

Effects of nonresonant ionization on multiphoton ionization line shapes

A kinetics model of multiphoton ionization (MPI) is developed to account for nonresonant ionization and collisional rethermalization effects on MPI. Simple quantitative criteria to describe MPI line shapes are derived. In cases where resonantly enhanced MPI (REMPI) is not an effective spectroscopic tool, it is demonstrated that absorption can deplete ground state levels which would otherwise have contributed to nonresonant ionization. The resulting structured valleys in the ionization current contain spectroscopic and dynamical information which may otherwise be unattainable. The feasibility of depletion of nonresonant ionization (DNRI) as a general technique is investigated and the first experimental observations of DNRI are reported and analyzed.

Picosecond Time-Resolved Photoacoustic Spectroscopy

In this paper we report the development and application of photoacoustic spectroscopy (PAS) for the detection and characterization of transient intermediates with lifetimes on the picosecond timescale [1].