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A multiplex infrared-visible sum-frequency spectrometer with wavelength tunability of the visible probe A new simple and low cost scattered transmission accessory for commercial double beam ultraviolet-visible spectrophotometers Rev.We describe two very long path absorption cells for sensitive laser absorption spectroscopy which are based on a design first described by Chernin and Barskaya ͓Appl. Opt. 30, 51 ͑1991͔͒. The cells distribute a nearly arbitrary number of spots across both dimensions of the field mirror in an efficient, compact array: confining up to 500 traversals in a 26 cm diam cooled cylindrical vacuum housing. The first cell, constructed at Denison University, is one of several multipass cells in the Laser Spectroscopy Laboratory used in the visible and near infrared ͑400-1100 nm͒. Optical paths of up to 1 km have been attained in this 3 m long cell at pressures from near vacuum to several atmospheres. The cell operates at temperatures down to 125 K. Conventional Pyrex optics are used. The second cell is part of a long path laboratory laser absorption spectrometer used in the thermal infrared: 350-2000 cm Ϫ1 at Goddard Space Flight Center. Path lengths up to 476 m have been achieved at frequencies less than 500 cm Ϫ1 . Two unique features of the Goddard cell are the piezoelectric micrometers, which simplify considerably the controls for the optical alignment, and the diamond-turned spherical mirrors, which significantly reduce the cost and the complexity of cryogenic operation. In addition, we discuss criteria for determining the optimal pathlength of a given multipass cell for the measurement of weak spectral lines.

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