Laser-induced, grating spectroscopy exploring new excited-state geometries

Spectroscopic techniques employing nonlinear optical methods continue to be developed and applied as sensitive probes of molecular reaction dynamics. A two-color variant of laser-induced grating spectroscopy has been configured to explore the multidimensional nature of electronically-excited polyatomic molecules in the gas phase. We employ double-resonant excitation pathways to control reagent geometry in a directly dissociative or predissociative photofragment event. Two pump beams are crossed at a small angle, interfering with each other to produce a spatially periodic light intensity in the crossing region. When this frequency is resonant with a transition, a sinusoidal modulation of the population of excited molecules is formed. Less obvious, is the presence of a depletion grating, formed by a sinusoidal modulation of the population of unexcited molecules. A third beam, of different frequency, is employed to probe either the excited state grating or the ground state grating. In the study of methylamine the probe beam was scattered off of an excited state grating. Methylamine electronic absorption spectra were extremely sensitive to the initial state of the molecule. Ground state depletion gratings were used to study chlorine dioxide. Excited state lifetimes were greatly affected by the initial state of the dissociating molecule. These two studies demonstrate the pivotal role initial nuclear geometry plays on photodissociation dynamics.