Laser-Induced Fluorescence/Dispersed Fluorescence (LIF/DF) Spectroscopy

Reactive chemical intermediates, especially free radicals, are of crucial importance to combustion and atmospheric chemistry. In our High-Resolution Laser Spectroscopy Lab, we use the laser-induced fluorescence/dispersed fluorescence (LIF/DF) technique to study the structure and dynamics of free radicals. They are usually produced under jet-cooled conditions (T_rotation~1 K)) using laser photolysis, discharge, pyrolysis, or laser ablation followed by free-radical reactions.

In the LIF experiment, molecules or free radicals are excited from the ground state to a high-energy state with a laser. Total fluorescence emission from the high-energy state to lower states is collected and detected. By scanning the frequency/wavelength of the laser, the energy-level structure of the higher-energy state can be mapped out. In the DF experiment, the frequency of the excitation laser is parked on a LIF transition, while the fluorescence is dispersed and detected with a spectrograph. The red-shift of the fluorescence signal with respect to the excitation frequency gives the lower-state energy-level structure. The intensities of the LIF and the DF transitions provide further information about the studied molecule, such as the geometries and symmetries of the upper and lower states.

In our experiment, free radicals are generated in a supersonic jet expansion. The seeded flow of precursor molecules was expanded through a pinhole valve into the vacuum chamber to reach a low temperature (~1 K). An excimer laser is used for photolyzing the precursors. Alternatively, a Nd:YAG laser can be used for photolysis (using its 355 nm third harmonic or 266 nm fourth harmonic) or ablation (using its 1064 nm fundamental) followed by free-radical radiations. A dye laser pumped by the second harmonic of a Nd:YAG laser is used to excite the LIF transitions. The laser-induced fluorescence was collected by a lens system perpendicular to both the excitation laser beam and the jet expansion and detected by a photomultiplier tube (PMT). In the DF experiment, the fluorescence is focused into and dispersed by a monochromator, and detected by an intensified charge-coupled device (CCD) camera.

Experimental Details

Laser-induced fluorescence (LIF) and dispersed fluorescence (DF) system for gas-phase laser spectroscopy. Free radicals are generated under jet-cooled conditions using laser photolysis, discharge, laser ablation, or pyrolysis. A fluorescence detection system with a photomultiplier tube (PMT, Hamamatsu, H10721-01) and a wide band amplifier (Hamamatsu, C6438-01) is used for LIF measurement, while a monochromator (Acton Research, SpectraPro 300i) with an intensified CCD camera (Princeton Instruments, PI-MAX 512) is used for DF measurement. A pulsed dye laser or OPO is used for recording vibronic spectra or survey scans. The pulsed dye laser is pumped with the second harmonic of a Nd:YAG laser. It has two gratings and can cover wavelength regions of 400 to 920 nm (with linewidth~0.1 cm^-1) and 370 to 620 nm (with linewidth~0.05 cm^-1), respectively. With the existing frequency doubling crystal, the frequency conversion unit (FCU) can cover 250-380 nm. The OPO is pumped with the third harmonic of a Nd:YAG laser. It is equipped with a frequency-doubling unit and covers a wavelength range of 205-2550 nm. It has a linewidth of ~5 cm^-1 and the pulse energy at the peak wavelength is ~40 mJ for the signal output and ~5 mJ for doubled signal output. A CW ring laser or CW-seeded laser amplifier is used for rotationally resolved high-resolution spectroscopy measurement. The monochromator used in the DF experiment has a focal length of 300 mm and a resolution of 0.1 nm @ 435.8 nm with 10-μm slit width. There are currently two gratings on the triple-grating turret.