Apparatus for the study of the IR multiple-photon resonances of polyatomic molecules via the optothermal technique. II. The superconducting bolometer

Abstract

A superconducting (sc) transition edge bolometer has been developed to detect the rovibrational energy gained in a multiple-photon absorption process. In this experiment, the polyatomic molecules previously cooled in a pulsed nozzle beam are irradiated by a pulsed IR laser (see Part I). The bolometer resistive element is a suitably patterned granular aluminium film deposited on a sapphire substrate. When the film is set at its operation point near the center of the sc transition (∼2 K), it exhibits an electrical resistance R∼290 Ω and a temperature derivative (dR/dT)∼2×104 Ω/K. A voltage responsivity R∼2×104 V/W and an effective time constant τe ≂60 ns (for excitation by radiation) are simultaneously obtained. The high values of the film resistance and of the responsivity allow a useful signal to be directly extracted, with a favorable signal-to-noise ratio, from the cryostat, thus avoiding the use of a cryoamplifier. An unconventional solution is used to compensate, without appreciable noise contamination, the dc voltage produced by the bolometer bias current (∼3×10−4 A). Although rather wide band amplification (0–8.3 MHz) is used to observe the signal produced by the laser irradiation of the molecular beam, in real time and without distortion, an input noise total power Pn ≂2.5×10−9 W is obtained for a single laser shot. A further reduction of the noise can be obtained by the acquisition and averaging system described in Part I.