Determining the dissociation threshold of ammonia trimers from action spectroscopy of small clusters

Abstract

Infrared-action spectroscopy of small ammonia clusters obtained by detecting ammonia fragments from vibrational predissociation provides an estimate of the dissociation energy of the trimer. The product detection uses resonance enhanced multiphoton ionization (REMPI) of individual rovibrational states of ammonia identified by simulations using a consistent set of ground-electronic-state spectroscopic constants in the PGOPHER program. Comparison of the infrared-action spectra to a less congested spectrum measured in He droplets [M. N. Slipchenko, B. G. Sartakov, A. F. Vilesov, and S. S. Xantheas, J. Phys. Chem. A 111, 7460 (2007)] identifies the contributions from the dimer and the trimer. The relative intensities of the dimer and trimer features in the infrared-action spectra depend on the amount of energy available for breaking the hydrogen bonds in the cluster, a quantity that depends on the energy content of the detected fragment. Infrared-action spectra for ammonia fragments with large amounts of internal energy have almost no trimer component because there is not enough energy available to break two bonds in the cyclic trimer. By contrast, infrared-action spectra for fragments with low amounts of internal energy have a substantial trimer component. Analyzing the trimer contribution quantitatively shows that fragmentation of the trimer into a monomer and dimer requires an energy of 1700 to 1800 cm(-1), a range that is consistent with several theoretical estimates.

Energetics and Predissociation Dynamics of Small Water, HCl, and Mixed HCl–Water Clusters

“…Infrared-action spectra of the dimer and trimer were obtained by detecting ammonia fragments from VP, and they provided an estimate of D 0 . 34 Following symmetric NH-stretch excitation in the clusters, excitation in the monomer fragment was detected only in the umbrella bending vibration (ν 2 ), and internal states populated up to the energetic maximum (ν 2 = 3 + ) of the dimer were observed. By monitoring individual rovibrational states of the ammonia monomer fragment by REMPI, 34 the contributions from the dimer and the trimer were identified.…”

“…Recently, Case et al used the same VMI technique described in section 3 to determine the D 0 for the ammonia dimer 29 and set limits on the D 0 of the ammonia trimer. 34 Experiments using infrared excitation of either the intramolecular symmetric NH stretch or antisymmetric NH stretch of the ammonia dimer in combination with VMI provided D 0 = 660 ± 20 cm −1 . 29 VMI studies using REMPI detection to monitor individual rovibrational states of one of the ammonia monomer fragments exhibited structured velocity distributions.…”

“…The experimental and theoretical research published since that time is described in detail in this Review. In recent years, the velocity map imaging (VMI) technique has been exploited successfully to determine accurate D 0 values for a considerable number of H-bonded dimers. − Recently, it has been extended to include the trimers of H 2 O, NH 3 , and HCl. − Moreover, the measured pair-correlated fragment state distributions, when combined with dynamics calculations, have helped elucidate dissociation mechanisms. ,− In fact, it is only with the help of theory that a unique interpretation could be achieved. The joint work on these clusters is highlighted in this Review, while examples of state-selected studies of VP of other small dimers and trimers carried out by other groups, although sometimes less comprehensive, are included for completion and comparison.…”

Conformational structures of 3-cyanoindole-(H2O) (n= 0–2): Franck–Condon simulations

“…Important structural information between the ground and excited electronic states can be obtained by Franck-Condon (FC) simulations [1,[5][6][7][8][9] that give rise to a reasonable test for the quality of calculated structures by comparing the experimental R2PI and/or UVHB spectrum. Since the vibronic transitions are highly sensitive to the FC integrals, the geometrical changes of the two electronic states are extremely reflected.…”

Infinite swapping in curved spaces

“…where dX = dx 1 dx 2 · · · dx N , P n is the permutation operator, and idnx(n, k) is the pointer to all the permutations in the coordinate-temperature list associated with temperature k. 24 The symbol x i is used to no-development of INS and PINS 24 is particularly timely, given our own interests in simulating molecular clusters, [26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41] and the difficulties we have encountered in the process. Molecular clusters seem to have a much richer set of rare-event like thermodynamic behaviors compared to an atomic cluster of equivalent size.…”