Picosecond Emission Spectroscopy with an Ultraviolet Sensitive Streak Camera

Abstract

Picosecond emission spectroscopy allows direct observation of rotational diffusion, of radiationless relaxation processes, and of energy transfer on the subnanosecond time scale. Risetimes are an indication of “slow” relaxation into the emitting state; while nonexponential decays result from “slowly” interconverting conformers, rotational diffusion, energy transfer, nonlinear processes, or solvent reorganization in a solvent-dependent fluorescing system. This paper will describe the use of picosecond pulses mainly from the 3rd and 4th harmonics of a Nd⁺³/glass mode-locked laser for ultraviolet excitation of molecular excited states. An ultraviolet sensitive ultrafast streak camera coupled to an optical multichannel analyzer interfaced with a computer serves as detector for light emission from these states. The time course of nonradiative events, which compete with the light emission, can often be obtained from the data. On the fastest time scales, one-photon ionization, when allowed, competes favorably with the well known electronic relaxation processes of internal conversion and intersystem crossing.