Calculated potential energy surfaces (PESs) for the ground and first electronically excited states of ammonia, as a function of H–NH2 bond distance (R H–NH2) and out-of-plane bending angle (ϕ) – see inset molecular structure for a definition of ϕ. PESs are obtained from Refs. 35 and 42 . After photoexcitation with hνpu to the ν′2 = 4 level of the state (squared wavefunction), adiabatic or non-adiabatic dissociation into either or photoproducts occurs via the state conical intersection (CI), as indicated by the blue and red arrows, respectively (see main text for further details).
TKER spectrum recorded after photoexcitation to the ν′2 = 4 level of the state at 200 nm with a pump-probe delay of t = 20 ps. Red ticks indicate the predicted TKERmax (see Eq. (1) ) for non-adiabatic dissociation into H-atoms in coincidence with rotationally cold radicals carrying up to 4 quanta in the ν2 bending mode. Inset: Enlarged portion of the TKER spectrum between 0 and 2200 cm−1, indicating the predicted TKERmax for adiabatic dissociation into the energetically open product channels (blue ticks). Blue shading schematically represents signal anticipated from adiabatic dynamics and is intended as a visual guide only.
False-color intensity plot of the TR-TKER spectrum between 0 and 3500 cm−1. Integral ranges for the feature A (0–400 cm−1) and feature B (600–1800 cm−1) signal transients in Figs. 4 and 6 , respectively, are also shown, together with the TKERmax values for adiabatic dissociation into the and photoproduct channels (white ticks).
Integrated H+ signal transient (between 0 and 400 cm−1) for feature A (open blue circles), correlated with the photoproduct channel. A kinetic fit to this transient with Eq. (3c) is shown by the solid blue line, while an attempted fit with Eq. (2) is given by the dashed blue line. Also presented is a NH3 + parent signal transient (black circles) obtained with a 200/243.1 nm pump/probe combination, with a kinetic fit to Eq. (3a) shown by the solid black line. An associated cross-correlation (XC) for these measurements (grey circles), attained through non-resonant multiphoton ionization of methanol, is also shown, fitted with a Gaussian IRF of 160 fs (grey line). Inset: H+ signal transient for the non-adiabatic photoproduct channel (open red squares) with an associated kinetic fit to Eq. (2) (solid red line).
Schematic of the step-wise first-order kinetics model proposed for adiabatic dissociation dynamics in ammonia, after photoexcitation to the electronically excited state (see main text for further details).
Integrated H+ signal transient (between 600 and 1800 cm−1) for feature B (open red circles), which contains signal associated with both and photoproduct channels. The solid red line serves as a visual guide only, and is generated from a constrained kinetic fit based on Eq. (3c) (see main text for details).
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