Rotationally resolved laser-excited

Experimental data pertinent to the vibrational predisaociation mechanism of two types of van der Waals complex are presented and discussed. First, variations in the infrared band shape for excitation of the ethylene out-of-plane wag, Vy, in the series of molecules CjH tHF, C2H Ne are discussed in terms of structure and relaxation mechanisms. Second, rotationally resolved laser excited fluorescence spectra for NeBr2 and NeCl2 are presented. There is a strong dependence of decay rate on molecular structure.. Relaxation... 

Next, some of the Si-Sq vibronic bands have been studied by us with rotationally resolved laser-induced fluorescence experiments, with the goal of providing a more precise determination of the BDO Si-state equilibrium structure and its deformation in the first vibrational excited levels. In this study, based on analysis of the rotational structure of the different vibronic bands, some problems with the vibrational assignment of the vibronic spectrum became immediately clear [18]. In fact, on the basis of previous studies some of the bands under investigation should have a level in common, either in the So or in the 5i state, but the experimentally determined rotational constants for those states were not equal within the experimental error. 

By the total internal reflection condition at the liquid-liquid interface, one can observe interfacial reaction in the evanescent layer, a very thin layer of a ca. 100 nm thickness. Fluorometry is an effective method for a sensitive detection of interfacial species and their dynamics [10]. Time-resolved laser spectrofluorometry is a powerful tool for the elucidation of rapid dynamic phenomena at the interface [11]. Time-resolved total reflection fluorometry can be used for the evaluation of rotational relaxation time and the viscosity of the interface [12]. Laser excitation can produce excited states of adsorbed compound. Thus, the triplet-triplet absorption of interfacial species was observed at the interface [13]. 

Using fs laser excitation at 620 nm, a 2PC in Y of 0.5ps [399] implicates hot electrons, probably thermalized at Te, as the mechanism for desorption induced by the fs laser (Section 2.6.2). Rotational state distributions are nearly Boltzmann characterized by Tf. The 2PC of internal state distributions was also obtained. Rather surprisingly, significant differences in these 2PC were obtained for T and the state-resolved yield for the two spin-orbit states and this was qualitatively rationalized by a DIMET picture [399]. Where overlap in experiments exist, the qualitative results are similar to those for fs laser induced desorption of NO/Pd(lll) [400,401]. For this latter system, the absolute yield Y is large at typical fluencies used in the experiments and a very hot vibrational distribution was observed (Tv = 2900 K). 

Figure 12. Level scheme of the rotationally resolved high-n Rydberg experiment. A first narrow-band laser pulse excites the molecule from the electronic ground state So into a single rotational state in the electronically excited S state. The frequency of the second laser pulse is scanned to obtain the rotationally resolved Rydberg spectrum shown in Fig. 13.

In order to make practical use of Child s semiclassical method, it is necessary to have absolute rotational assignments for the sharp lines. This is a nontrivial problem since the usual rotation-vibration branch and band structures are shattered beyond recognition. However, the sharpest lines will have appreciable fluorescence quantum yields, thus enabling rotational assignments to be made on the basis of resolved R(J — 1) - P(J +1) = B"(4J + 2) splittings in laser-excited fluorescence spectra. It is fortunate that the theoretical analysis is based on the sort of assignment information that is most accessible in the simplest tunable laser experiment. 

Typically, reaction mixtures were either flash photolyzed or laser excited under conditions which led to the production of the species of interest in the presence of the CO laser beam, preset to the various vibrational-rotational lines accessible to the reaction. The initial vibrational population distribution of the CO formed in the reaction was determined from time-resolved absorption curves measured for all vibrational levels populated by the reaction under study. The P(10) lines were used for most experiments because of their high output by the probing laser. However, no significantly different population distributions were obtained when other nearby lines were used randomly. 

Several recent experimental studies in small hydride molecules have indeed demonstrated that molecular rotation is instrumental in accepting the energy. Thus the relaxation in the 4 11 state of NH and ND was examined using pulsed UV laser excitation and time-resolved fluorescence studies. The data showed that, contrary to the energy-gap law predictions, NH relaxes order of magnitude faster than ND. Very similar behavior was observed in the state of OH in solid Ne. ... 

Fig. 10 Laser excitation spectrum of the vinoxy radical showing resolved rotational structure, with the assignments as indicated.

The rotationally resolved excitation spectrum, recorded on the R(I) line of the a X Z" emission while scanning the laser across the 0 — 0 band in the range 473 to 470 nm, is that of a nearly free rotor it consists of a sharp P(O) line at 472.72 nm, an electric-dipole-forbidden but electric-quadrupole-allowed Q(0) line at 472.05 nm, and a crystal-field-split R(O) line around 470.5 nm [9]. The sharp P(O) line in the excitation spectra of NH and ND in Ar or Kr matrices has been the subject of a systematic study on the temperature... 

There are two approaches for time-resolved RR spectroscopy of these retinal proteins. Using cw-excitation a time-resolution down to 100 ns can be achieved by rapidly moving the sample through the laser focus (rotating cell, capillar flow system). Pulsed laser excitation can provide a time-resolution even in the subpicosecond range depending on the... 

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