Picosecond rotationally resolved stimulated emission pumping spectroscopy of nitric oxide

Abstract

Stimulated emission pumping (SEP) experiments were performed on the nitric oxide molecule in a flow cell environment using lasers with pulse widths of 17–25 ps. A lambda excitation scheme, or ‘‘pump–dump” arrangement, was employed with the pump laser tuned to the T 00 vibronic band origin ( λ pump = 226.35 ( 1 ) nm ) of the A 2 Σ +( v′ = 0, J′) ← X 2 Π 1/2( v″ = 0, J″) and the dump laser scanned from 246–248 nm within the A 2 Σ +( v′ = 0, J′) → X 2 Π 1/2( v″ = 2, J″) transition. The rotationally resolved SEP spectra were measured by observing the total fluorescence within the A 2 Σ +( v′ = 0, J′) → X 2 Π 1/2( v″ = 1, J″) transition between 235 nm and 237.2 nm while scanning the dump laser wavelengths. Multiple rotational states were excited due to the broad laser bandwidth. Measurements showed that the resolved rotational structure depended on the energy and bandwidth of the applied pump and dump laser pulses. Analysis of the observed fluorescence depletion signals yielded an average percent fluorescence depletion of about 19% when λ pump = 226.35 ( 1 ) nm and λ dump = 247.91 ( 1 ) nm . This value reflects the percent transfer of the NO population from the A 2 Σ +( V′ = 0, J′) excited electronic state to the X 2 Π 1/2( v″ = 2, J″) ground electronic state. The maximum expected depletion is 50% in the limit of dump saturation. Selective excitation of NO at the bandhead provides good spectral discrimination from the background emission and noise and unambiguously confirms the identity of the emitter.