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Absorption Spectroscopy and Its Implementation
Published in Helmut H. Telle, Ángel González Ureña, Laser Spectroscopy and Laser Imaging, 2018
Helmut H. Telle, Ángel González Ureña
For molecules, a method different from MPI can be utilized, in principle, in which the additional photon results in molecular dissociation rather than in ionization. When investigating organic photochemical mechanisms in the late 1960s and early 1970s, visible luminescence was observed when high-power (IR) laser radiation interacted resonantly with absorption bands of polyatomic molecules, such as ammonia (NH3), boron-trichloride (BCl3), and chloro-trifluoroethane (C2F3Cl)—to name but a few examples. In these and all other cases, it was found that the luminescence threshold was lower than the optical breakdown (i.e., a plasma with charged particles ensues) threshold; the analysis of the luminescence spectra revealed that they were associated with emission from molecular dissociation products. Indications were that the underlying process could only be due to the simultaneous absorption of (many) IR photons—typically of the order of 20–30, or more—depending on the particular bond dissociation and photon energies. The process is normally called infrared multiphoton dissociation (IR-MPD).
Cryokinetics and spin quenching in the N2 adsorption onto rhodium cluster cations
Published in Molecular Physics, 2021
Amelie A. Ehrhard, Matthias P. Klein, Jennifer Mohrbach, Sebastian Dillinger, Gereon Niedner-Schatteburg
IR spectroscopy of rhodium clusters. IRMPD (infrared multiphoton dissociation) spectroscopy of [Rhn(CO)x]-/0/+ clusters revealed novel correlations: Observed CO stretching band shifts correlate with the size of the cluster core and are interpretated through a ‘charge dilution model’ [37–40]. Subsequent IRMPD studies confirmed N2O reduction by Rh4-8+ clusters [41]. Far-IR-MPD studies of naked Rh6-12+ clusters served to verify DFT model calculations that predicted tetrahedral and octahedral structures rather than cubic ones [42]. This prediction found support through analogous findings for neutral and anionic clusters [43].
Vibrationally resolved electronic spectroscopy of rhodamine B cation at less than 5 K: combining gas phase absorption and matrix isolation fluorescence spectroscopy
Published in Molecular Physics, 2023
Sreekanta Debnath, Alexander Schäfer, Karolina A. Haupa, Dmitry Strelnikov, Jana Roithová, Juraj Jašík, Sergei Lebedkin, Manfred M. Kappes
In this context, due to reduced complexity in the presence of a negligible perturbing environment, gas-phase action spectroscopy of mass-selected ions provides an ideal arena to explore fundamental photophysical properties of chromophores at a strictly molecular level [6–13]. Several different types of gas-phase action spectroscopy are known and have been widely applied to the study of molecular ions. They include anion photoelectron spectroscopy, resonance-enhanced multiphoton ionisation, IR-UV two-color ionisation, photoluminescence (excitation and resolved emission), one-photon photodissociation/photodepletion, infrared multiphoton dissociation, IR-UV photodissociation and messenger-tagging photodissociation spectroscopies. The latter arguably provides the best approach to obtaining spectra of one photon absorption cross sections [14]. Here, a weakly bound ion-messenger (AB+/–•M) complex is formed by weak electrostatic interaction between the ion of interest (AB+/–) and the messenger M, typically a rare gas atom (He, Ar) or an inert molecule (D2, N2). In the case of cationic systems, He is considered to be the best choice for the messenger as it induces negligible structural perturbation, i.e. structural and electronic properties obtained for the complex can be approximated to the properties for the bare ion [15]. As a corollary, the ions of interest have to be cooled to low enough vibrational temperatures (typically 5 K or lower) to adsorb the helium tag and retain it on the experimental timescale. Consequently, photodissociation (PD) spectra recorded using this technique can manifest well-resolved vibronic structure with correspondingly narrow linewidths even for relatively large molecular ions with more than 50 atoms – also owing to comparatively slow relaxation rates.
Quantum dynamics and spectra of the iodine atom in a strong laser field as calculated with the URIMIR package
Published in Molecular Physics, 2019
R. Marquardt, M. Quack, J. Stohner, I. Thanopulos
The present work has as focus the coherent radiative electronic excitation in the iodine atom treated as a multilevel system using the current extensions of the URIMIR package (see the Appendix). The work is motivated by considering properties of the magnetic dipole transition in the ground configuration, which is among other things also the basis for the iodine atom laser [40]. This transition has been the subject of accurate measurements of the hyperfine structure and rather late also accurate determination of its absolute transition strength [41,42]. It has been used for exceptional kinetic measurements combining rates with the observation of product translational energy distributions after infrared multiphoton dissociation of organic iodides and including at the same time observation of the hyperfine distribution in the iodine atom product at what can be called ‘uncertainty limited’ kinetic measurements at highest time and energy resolution, only limited by the Heisenberg uncertainty principle [43,44]. This can be considered in some sense to be the ultimate limit of ‘kinetic spectroscopy’ as a method of time resolved spectroscopic measurement of reactants or products in reaction kinetics [45]. In these experiments involving at the same time intense CO laser excitation (in the wavenumber range 900–1100 cm−1 and with intensities in the 100 MWcm−2 range) and diode laser probing at 7600 cm−1 with low intensity, the question of the effect of the simultaneous laser irradiations arose, in particular as far as shifts and splittings in the hyperfine spectra are concerned. These can be estimated by perturbation theory [46], but because of the usual uncontrolled uncertainties in perturbation theory results a more fundamental theoretical test appeared to be important. We thus report here extensive numerical calculations using the extended URIMIR package, which we publish as supplementary material as well. It turns out that the perturbation theory is rather well confirmed, anticipating some of our results. We might mention here also lower resolution measurements of iodine atoms produced by photolysis of organic iodides in solid parahydrogen [47] using this fine structure transition with only partly resolved hyperfine structure.