Contribution of the Gouy phase to two-pathway coherent control of the photoionization and photodissociation of vinyl chloride

Abstract

The electric field of a light wave accumulates a pi phase shift as it passes through a focus. We show here how this effect, known as the Gouy phase, may be used to control the branching ratio of a unimolecular reaction when the products are formed with different numbers of photons. We demonstrate this control method for the ionization and dissociation of vinyl chloride, using absorption of 177 and 532 nm photons to induce a pair of interfering paths. Excellent agreement between the observed and calculated phase shift as a function of the axial coordinate of the laser focus indicates that fragmentation occurs via a ladder switching mechanism. The axial dependence of the modulation depth is evidence of loss of coherence at higher internal temperatures of the molecule.

Use of the spatial phase of a focused laser beam to yield mechanistic information about photo-induced chemical reactions

“…The phase lag between the acetyl and acetone ions obtained at five values of z m (the 'Gouy profile') is plotted in figure 3. Also shown in this figure is the previously published [3] Gouy profile for VCl, which is seen to be very similar to that of acetone.…”

“…Peaks that showed strong modulation are CH . The vinyl chloride data are taken from [3]. The solid curve is the numerical calculation of the spatial phase lag using the methods described in [4], taking into account the astigmatic focus of the laser beam and the Gaussian profile of the molecular beam.…”

“…The experimental setup was described previously [3]. Briefly, a pulsed molecular beam of the neat sample gas was simultaneously excited with 532 and 177 nm radiation.…”

“…In such a situation, relatively little dynamical information is learned from the phase lag. In a recent series of papers, however, we showed that if formation of some of the products requires the absorption of additional photons after the control step, it is possible to extract information from the phase lag about the reaction paths leading to those products [3]- [5]. Consider the case of bichromatic excitation by n photons of frequency ω m and m photons of frequency ω n , such that nω m = mω n .…”

“…In a recent study [3], we demonstrated a Gouy phase contribution to the branching ratio of the photoionization versus photodissociation of vinyl chloride (CH 2 CHCl, VCl) in threeversus one-photon excitation at 532 and 177 nm. Phase control of the reaction products for this molecule is of interest because it shows that the high density of states of a multicenter polyatomic molecule need not destroy the coherence effects that were previously observed for simpler molecules.…”