Fluorescence collisionally induced

Two-photon absorption is detected by observing collisionally induced Lyman a fluorescence in a cell containing atomic hydrogen. This experiment has been set up and we shall report on the results soon. 

One example of this pump-and-probe technique is the investigation of collision-induced vibrational-rotational transitions in the different isotopes HDCO and D2CO of formaldehyde by an infrared-UV double resonance [1041]. A CO2 laser pumpes the V6 vibration of the molecule (Fig. 8.18). The collisional transfer into other vibrational modes is monitored by the fluorescence intensity induced by a tunable UV dye laser with variable time delay. 

Crosley D R 1981 Collisional effects on laser-induced fluorescence Opt. Eng. 20 511-21... 

Duncan M A, Bierbaum V M, Ellison G B and Leone S R 1983 Laser-induced fluorescence studies of ion collisional excitation in a drift field rotational excitation of N in He J. Chem. Phys. 79 5448-56... 

Laser-induced fluorescence (LIF). Laser-induced fluorescence measurements have been applied to the atmosphere since the suggestion of Baardsen and Ter-hune in 1972 that this method should be feasible. Figure 11.43 shows the energy levels and transitions involved in LIF measurements. OH is excited from its ground X2n state into the first electronically excited A22 state. The v" = 0 to r = 0 transition is around 308 nm and the v" = 0 to v = 1 at 282 nm. Two schemes have been used excitation using 282 nm into v = 1 of the upper electronic state, or excitation using 308 nm into v = 0 of the upper state. Collisional quenching deactivates some of the v = 1 into u = 0 in competition with fluorescence, mainly in the (1,1) band of the electronic transition (that is, from v = 1 of the upper state into v" =1 of the lower state). Collisional deactivation of v = 0 then occurs in competition with fluorescence in the (0,0) band at 308 nm... 

Several years ago, Flory and Johnston141 excited the first-vibrational level of the C2II state with radiation at 1832 A from a mercury arc. They used NO pressures between 0.02 and 7 torr, found N2 and Oa as products, and monitored the initial pressure drop induced by exposure. The pressure drop was proportional to the absorbed intensity. Thus, as they pointed out141,142 photochemical results require predissociation to be the primary process, because the pressures used were so low that collisional processes would be negligible compared to fluorescence. 

One of the main problems met in Laser Induced Fluorescence measurements is the excited population dependence on the quenching due to collisional deexcitation. The saturation mode proposed to avoid this dependence is very difficult to achieve U ) (2 ) particularly with molecular species and moreover the very strong laser pulses required may cancel the non-perturbing characteristic of the method. Therefore precise knowledge of the quenching is necessary in some experimental circumstances. 

Experiments using the technique of laser-induced fluorescence (LIF) in flames have provided ample demonstration of its selectivity and sensitivity, and hence of its applicability as a probe for the reactive intermediates present in combustion systems. The relationship between the measured fluorescence intensity and the concentration of the molecule probed, however, must take into account the collisional quenching of the electronically excited state pumped by the laser. Because the flame contains a mixture of species, each with different quenching cross sections, it may be difficult to estimate the total quenching rate even if many of these cross sections are known. 

So far we have discussed various studies on the relaxation of the population bPo of excited state rovibrational levels in elastic and inelastic collisions. To this end the intensity of fluorescence was measured in one or the other way. If an analysis of the state of polarization of the radiation is performed, one may obtain information on the behavior of alignment and orientation of the molecular angular momenta in elastic and inelastic collisions. If we register, under collisional conditions, the polarization properties of a directly laser-induced rovibrational level of the molecule, then, according to (2.35) and (2.39), it is possible to determine the rates... 

I.r. fluorescence from Fe(CO)5 at 4, 5, and 16 pm was monitored following C02-laser excitation in the 10 pm region.The observed energy transfer behaviour was discussed in terms of theories of V-V and V-T/R transfer and collisional energy transfer. The isomerization of Fe(CO)i induced by i.r.-laser radiation has been studied by Poliakoff and Ceulemans." " A topological model was developed to rationalize the observed behaviour. 

The three non-adiabatic effects most easily studied by beam techniques are (1) collisional excitation followed by the emission of radiation for the upper state. (2) collisional ionization to A+ + B-, (3) Penning ionization. Collision induced fluorescence is discussed by V. Kempter in Article 9 the technique is a relatively sensitive one and cross sections down to 10-21 cm2 can be detected, though this still falls short of the sensitivity of bulb spectroscopic methods. The technique is also powerful in that separate cross sections for excitation of members of a multiplet can be obtained although, as always with a beam technique, absolute values are difficult to obtain. 

Although flames are convenient sources of MOH molecules, they suffer from serious drawbacks for spectroscopic and dynamical studies. The high temperature ( 2000 K) of flames causes numerous vibrational and rotational levels to be populated resulting in very dense spectra. The high pressure (1 atm) broadens the rotational lines (>0.1 cm ) and increases the overlap of the lines. In addition, resonant laser-induced fluorescence is difficult to detect because of quenching and the overwhelming presence of nonresonant fluorescence caused by rapid collisional energy transfer. The luminescence of the flame itself also interferes with measurements. 

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