New Spectral Techniques: Time‐Resolved Fourier‐Transform Spectroscopy and Two‐Color Laser‐Induced Grating Spectroscopy

Time‐resolved Fourier‐transform spectroscopy and two‐color laser‐induced grating spectroscopy are two new techniques recently employed in this laboratory. We recorded emission in the near infrared region during laser photolysis of HONO₂ with a step‐scan Fourier‐transform spectrometer and achieved temporal resolution in the microsecond range and spectral resolution of 0.1 cm¹. Rotationally resolved emission lines of the (0,0) band of the D ²∑⁺ →A ²∑ ⁺ transition of NO in the region 8900‐9300 cnv⁻¹ with irregular relative intensities were observed when an ArF excimer laser was used to photodissociate HONO₂. The spectroscopic parameters of both D ²∑⁺ and A ²∑⁺ states agree with those previously reported. When a narrow‐band ArF laser was used, selective rotational levels of the D state of NO were populated depending on the wavelength of the ArF laser. Our results indicate that absorption of a 193‐nm photon by NO(υ″ = 1) is responsible for the observed emission. To test the technique of two‐color laser‐induced grating spectroscopy, we employed the B ³II_0U ⁺‐X ¹∑_g ⁺ system of I₂. Background‐free spectra with transitions involving rotationally selected states were recorded. Various experimental schemes were employed with population gratings formed in either the B or X state. Signals due to different four‐wave mixing schemes were distinguished by variation of relative timing between the grating beams and the probe beam.