John Polanyi

Aligning Molecules and Zapping them with Lasers: How and why


Abstract

Over the past fifteen years we have developed an approach to the study of ordered chemical dynamics in two dimensions that we term 'Surface Aligned Photochemistry', SAP; an adsorbate layer is ordered by an underlying crystalline substrate prior to photolysis of the adsorbate. This gives significant and selectable control of photofragment collision angles and impact parameters.

Examples will be given in which the formation of photofragments is directed (a) upward away from the surface (b) along the surface toward a coadsorbate, and (c) downward at the surface leading (in cases (b) and (c)) to 'localised' encounters with the co-adsorbate and substrate. Cases (a) and (b) will be exemplified by further recent work on CH3Br(ad) photolysed at 193nm on LiF, NaCI and MgO single-crystal substrates. Methyl radicals leave the surface by two resolvable, controllable pathways (a) 'direct' and (b)'indirect'. The variable dynamics were confirmed as being due to varied adsorbate geometry by means of polarized Fourier Transform Infrared Spectroscopy performed in UHV.

The case of downward recoil ((c)) will be exemplified first by recent work on halides adsorbed on LiF, in which the scattered H which is involved in localised Atomic Scattering (LAS) at specified atoms on the surface is interrogated by two-colour laser excitation to a high Rydberg state (the K. Welge approach, used here for the first time for UHV studies). The recoiling H can, in certain cases, exhibit quantisation of its translational motion corresponding to vibration in the collision partner.

Secondly, and importantly, the case of downward recoil ((c)) will be illustrated by recent studies in our laboratory in which halides adsorbed on Si( 111) give rise to Localised Atomic Reaction (LAR) at the surface. The photoreactions are followed, at the atomic level, by Scanning Tunneling Microscopy. This new phenomenon of LAR may have utility in photoimprinting patterns of molecular dimensions on Si.

Finally, recent work will be described in which the reagents are aligned and ordered prior to photoreaction by the formation of gaseous complexes (cf. B. Soep, C. Wittig, D. Neumark, and A. Zewail) with the consequence that photoinduced reaction proceeds through selected transition states (TS). A brief account will be given of recent experimental and ab initio theoretical results from this laboratory in which this approach is used for the first time to explore the TS of the classic alkali-metal harpooning reactions induced by the effect of tunable visible light on the gaseous complexes NA..FH, Na..FCH3, Na..CICH3 and Na..BrCH3 formed at the crossing point between molecular beams of the constituents Na and RX. All these complexes upon irradiation can yield Na+X- + R via a TS which we write as: [Na*..XR ~ Na+..XxR-]. Our objective examine the harpooning reaction dynamics as a function of the geometry of the transition state- an example of Transition State Spectroscopy.


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