Collision induced satellites

Fig. 8. Collision-induced satellite spectrum of Naj around X = 5290 A, due to fluorescence originating from rotational levels populated by collision from the V = 6, / = 43 level in the state, which was excited by the X = 4880 A argon laser line. (From ref.

Fig. 8.7 Collision-induced satellite lines Q(AJ ), R(A/ ) and P(A7 ) in the LIE spectrum of Na2 under excitation of the rotational level B FIuiv = 6, J = 43), The parent line is recorded with one-twentieth the sensitivity. The high satellite lines are related to Au = 0 transitions and partly superimpose the weaker satellites from Av = 1 collision-induced transitions [987]...

Molecules in the collisionally populated levels m) can decay by the emission of fluorescence or by further collisions. In the LIF spectrum new lines then appear besides the parent lines, which are emitted from the optically pumped level (Fig. 8.7). These new lines, called collision-induced satellites contain the complete information on the collision process that has generated them. Their wavelength X allows the assignment of the upper level m) = + An, 7 + A7), their intensities yield... 

Fig.l0.31a,b. A-type and V-type OODR signals of Cs2- (a) Level scheme and (b) OODR signals. The signals marked with x are collisions-induced satellite signals. The frequency markers are 60 MHz apart... 

J" = 28 - J = 28 1) appear as double resonance signals while at 0.1 torr collision-induced satellites are generated... 

Fig. 12.7. Collision-induced satellite lines in the laser-induced fluorescence spectrum. Example of rotational and vibrational transitions from the (v =6,J =43) level in the state of Na2- The parent line is 20-fold off scale...

The Intensity ratio between these satellite lines" and the main fluorescence line yields the absolute cross sections for the different collision-induced rotational transitions, provided the He pressure and the lifetime of the excited Na state are known. Both parameters have been measured, giving cross sections between = 1.5 A for A/ = 1 transitions and 6 = 0.3 A for A/ = 5... 

Information concerning the interaction potentials of Cs (7S, 5D)-rare gas pairs is obtained by interpreting the temperature dependence of the 6S-7S,5D far wing and satellite profiles. A sensitive laser fluorescence technique is used to obtain the absorption coefficient of the mixture. The collision induced oscillator strength, a rapidly varying function of the interatomic distance in the case of such forbidden transitions, is also deduced. Experimental potentials and oscillator strengths are compared with available calculated values. 

Such determinations of rotationally inelastic integral cross sections ajcm for collision-induced transitions in excited molecules obtained from measurements of satellite lines in the fluorescence spectrum have been reported for a large variety of different molecules, such as I2 [983, 984], Li2 [985, 986], Na2 [987], or NaK [988]. For illustration, the cross section a (A7) for the transition 7 7 + A7 in excited NaJ molecules induced by collisions Na + He are plotted in Fig. 8.8. They rapidly decrease from a value a(AJ = 1) 0.3 nm to a(AJ = 8) 0.02 nm. This decrease is essentially due to energy and momentum conservation, since the energy difference AE = E(J A7) — E(J) has to be transferred into the kinetic energy of the collision partners. The probability for this energy transfer is proportional to the Boltzmann factor exp[—A /(A r)] [989]. 

In addition to LIF resonant two-photon ionization (Sect. 1.4) can also be used for the sensitive detection of collision-induced rotational transitions. This method represents an efficient alternative to LIF for those electronic states that do not emit detectable fluorescence because there are no allowed optical transitions into lower states. An illustrative example is the detailed investigation of inelastic collisions between excited N2 molecules and different collision partners [995]. A vibration-rotation level (v, J ) in the a Jig state of N2 is selectively populated by two-photon absorption (Fig. 8.10). The collision-induced transitions to other levels v + An, / + AJ) are monitored by resonant two-photon ionization (REMPI, Sect. 1.2) with a pulsed dye laser. The achievable good signal-to-noise ratio is demonstrated by the collisional satellite spectrum in Fig. 8.10b, where the optically pumped level was v = 2, J = 7). This level is ionized by the P(l) parent line in the spectrum, which has the signal height 7.25 on the scale of Fig. 8.10b. 

While collision-induced transitions in excited electronic states can be monitored through the satellite lines in the fluorescence spectrum (Sect. 8.2.2), inelastic collisional transfer in electronic ground states of molecules can be studied by changes in the absorption spectrum. This technique is particularly advantageous if the radiative lifetimes of the investigated rotational-vibrational levels are so long that fluorescence detection fails because of intensity problems. 

The orientation of the molecules due to saturation by a polarized pump wave can be partly transferred to neighbouring levels by inelastic collisions of the oriented molecules with other atoms or molecules. This produces "satellite lines" in the double-resonance spectrum which may partly diminish the unambiguity of line assignment. Some of the weaker lines in Fig.10.43 are such collision-induced polarization signals. 

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