Infrared Multiphoton Induced Isomerization of cis‐3,4‐Dichlorocyclobutene. III. Pressure Dependence of the Yield

Abstract

AbstractThe multiphoton induced isomerization of cis‐3,4‐dichlorocyclobutene (DCCB) was studied as a function of pressure. Three experiments indicated the existence of a nonthermal mechanism at neat pressures below 60 mtorr. These were: (i) the leveling off of the fractional yield of isomerization products at low pressures; (ii) the absence of dissociation products from 2‐iodo‐2‐methyl propane, which served as a chemical thermometer in the presence of a large excess of DCCB; and (iii) a break in the power‐law dependence of the optoacoustic signal produced by neat DCCB. The increased yield at higher DCCB pressures is believed to be due partly to a thermal reaction and partly to increased absorption caused by rotational or vibrational hole filling. Optoacoustic measurements also showed that at constant fluence the absorbed energy was greater for long, low intensity pulses, as might be expected for inhomogeneous excitation. However, the present experiments did not reveal an intensity dependence of the reaction yield. Addition of argon as a buffer gas monotonically decreased the yield, indicating that vibrational quenching more than offsets any hole‐filling effects.