A continuous wave photofragment spectrometer

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Abstract
A continuous wave photofragment spectrometer is described which can investigate photodissociative reactions by measuring angular dependent mass spectra of the recoiling photofragments. The three‐state differentially pumped vacuum system consists of three mutually orthogonal axes: a polarized light axis (high pressure Hg–Xe), a molecular beam axis, and a quadrupole mass filter axis. The electric vector of the polarized light is rotated by a stepper motor (1‐rpm). The photofragment signal from the tuned amplifier is lock‐in detected and displayed on a strip chart recorder. Various inorganic filter solutions are employed in order to suitably excite the desired transitions of molecules with favorable absorption coefficients (∼100 1 mole−1 cm−1) in the spectral range of the excitation source. Signal averaging is accomplished using long lock‐in amplifier time constants as well as multichannel scaling. The relative merits of cw photofragment spectroscopy are also discussed with particular reference to the photodissociation of large aromatic molecules. From the angular data, the polarization of the transition dipole relative to the detector axis, the lifetime of the excited molecular state, and the relative ease of fragmentation into different channels can be determined.

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