Optoacoustic measurements of IR multiphoton excitation of cis-3, 4-dichlorocyclobutene

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Abstract
Optoacoustic detection was used to study the infrared multiphoton excitation of cis‐3,4‐dichlorocyclobutene (DCCB). The dependence of the optoacoustic signal SOA on DCCB pressure PDCCB, on Ar buffer gas pressure PAr, and on laser fluence φ were studied systematically. It was observed that SOA is proportional to PDCCB and increases monotonically with PAr, leveling off at PAr∼10 Torr. Absolute calibration of SOA was achieved by measuring the attenuation of the laser beam. It was found that the average number of photons absorbed by DCCB n̄ varies as 5.4 φ0.85, where φ is expressed in J/cm2. It is shown that the buffer gas pressure dependence of SOA can be used to deduce the vibrational relaxation rate of the excited molecule. The fluence dependence of n̄ is predicted quantitatively by numerical solution of the master equation, including collision terms. It is assumed in this calculation that the infrared absorption cross section is independent of vibrational energy and is equal to the nominal small signal cross section.