The origin of monomer cation upon 266 nm photoexcitation of nitrogenated (poly) aromatic heterocycle compounds is attributed to the dissociative photoionization of dimers based on time-of-flight mass spectrometry combined with velocity map ion imaging experiments [1]. The experiments performed in the presence and absence of buffer gas with varying laser intensities suggest that during neat expansion, monomer cations are formed via two-photon dependent resonantly enhanced multiphoton ionization (REMPI) while in the presence of buffer gas, (cluster formation enabled) broad translational energy distribution and three-photon dependence of monomers indicate dissociative photoionization of dimers as the origin. Interestingly, the REMPI of monomers appears at higher laser intensities in contrast to the three-photon dependent dissociative ionization of dimers that can occur even at lower laser intensities [2]. This contradicts the fact that the monomer concentrations are larger than those of clusters in the molecular beam and that the three-photon absorption should be less probable than two-photon absorption at lower intensities. These experimental results demonstrate that the interaction between monomer units in molecular clusters gives rise to cooperative/collective excitation phenomena that modifies the absorption cross-section, thereby enhancing multiphoton absorption compared to isolated monomers [3,4].
References:
[1] J.T. Snodgrass, R.C. Dunbar, M.T. Bowers. J. Phys. Chem. 94 3648–3651 (1990).
[2] S. Kesari, A. Tagad, G.N. Patwari, https://arxiv.org/abs/2408.02981 (2024).
[3] A.C. Laforge, M. Drabbels, N.B. Brauer, M. Coreno, M. Devetta, M. Di Fraia, P. Finetti, C. Grazioli, R. Katzy, V. Lyamayev, T. Mazza, M. Mudrich, P. O’Keeffe, Y. Ovcharenko, P. Piseri, O. Plekan, K.C. Prince, R. Richter, S. Stranges, C. Callegari, T. Möller, F. Stienkemeier. Sci. Rep. 4, srep03621 (2014).
[4] F. Šanda, S. Mukamel. J. Chem. Phys. 124, 124103 (2006).