The Value of Different Experimental Observables: A Transient Absorption Study of the Ultraviolet Excitation Dynamics Operating in Nitrobenzene

Excess energy redistribution dynamics operating in nitrobenzeneunder hexane and isopropanol solvation were investigated using ultrafasttransient absorption spectroscopy (TAS) with a 267 nm pump and a 340-750nm white light continuum probe. The use of a nonpolar hexane solventprovides a proxy to the gas-phase environment, and the findings aredirectly compared with a recent time-resolved photoelectron imaging(TRPEI) study on nitrobenzene using the same excitation wavelength[L. Saalbach et al., J. Phys. Chem. A 2021, 125, 7174-7184].Of note is the observation of a 1/e lifetime of 3.5-6.7ps in the TAS data that was absent in the TRPEI measurements. Thisis interpreted as a dynamical signature of the T-2 statein nitrobenzene Ranalogous to observations in the related nitronaphthalenesystem, and additionally supported by previous quantum chemistry calculations.The discrepancy between the TAS and TRPEI measurements is discussed,with the overall findings providing an example of how different spectroscopictechniques can exhibit varying sensitivity to specific steps alongthe overall reaction coordinate connecting reactants to photoproducts.