Electronic three wave mixing spectra of transient species produced by UV laser photolysis of benzene

Abstract

Intense three wave mixing (3WM) spectra are reported for transient fragments produced by 266 nm laser photolysis of benzene and several substituted benzenes. Single pulse broadband 3WM spectra taken with an optical multichannel analyzer establish that the fragments are primary photoproducts obtained under collision-free condition. The spectra consist of many features at anti-Stokes frequency shifts of 900–3100 cm−1 from a 532 nm ω1 pump. 90° fluorescence studies of the photolysis zone show that C2 is produced in various electronic states and energetic consideration require that dissociation of C6H6 must involve two or more photons at 266 nm. 3WM spectra of C6D6 are identical to those of C6H6 in the anti-Stokes shift region near 3000 cm−1 and hence the transients do not contain CH bonds. 3WM spectra of C2H2 fragments are quite similar to those of benzene in the 3000 cm−1 region so that C2 is believed to be responsible for both 3WM and fluorescence spectra. The 3WM spectra cannot be interpreted in terms of simple CARS vibrational resonances of C2. Intensity considerations suggest that enhancement due to multiple resonance is likely, and various electronic–electronic and vibrational–electronic 3WM processes are discussed.

Laser diagnostics and minor species detection in combustion using resonant four-wave mixing

“…the diatomic species C 2 [147][148][149], CH [150], and NO [151,152]. Furthermore polyatomic molecules such as NO 2 [153], NH 2 [154], and C 2 H 2 [155] have been investigated using this technique.…”

Optical detection of benzene fragmentation in IR and visible-UV laser-induced processes

“…In fact, we have observed that the broad feature at 4685/~, related to Swan's band transitions from the a3II~ vibrationally excited states above ~ 4000cm -1, increased more than all the other peaks measured in the range (4580 + 6100)/~. The later features were tentatively assigned by Gross et al [7] to resonance-enhanced CARS from highly excited vibrational levels of the AllIu C~ state lying about 15000cm -1 above the ground state. Present results show that colinear CARS is a simple technique for contemporary on-line detection of C6H6 and of its C2 fragments in laser-induced dissociation processes.…”

“…Most of the fragment peaks shown in fig. 2b) and listed in table II have also been observed at lower pressures and photolysis laser powers by [7] in the UV photodissociation of C6 H6 at 266 nm. These features are there discussed as probably related to resonantly enhanced CARS signals coupling different electronic states of the C2 photofragment.…”

“…In agreement with ref. [7], where the same structure has been detected in a two-photon UV photolysis of C6 H6 at 266nm, we assign this structure to some transitions in the Swan's band (d3IIg--~ a3H~) of C2 photofragment. The richest structure in the investigated spectral range is observed when the interaction of the C02 laser with C6 H6 occurs in the presence of the broad-band dye laser (see fig.…”

“…In the UV photolysis discussed in [7] the absence of ionic species is hypothesized and collisionless conditions are claimed for the formation of C2 in different electronic states. C2 in these states shows ( fig.…”