TE01	INVITED TALK	30min1:30

JOHN HALL, JILA, NIST \& Dept of Chemistry \& Biochemistry, Univ. of Colorado, Boulder, CO 80309.

High sensitivity detection of quantum absorption is demonstrated at a sensitivity of 1 x 10¹¹ integrated absorption, in 1 sec, using ~10 mTorr total gas pressure. Achieving highly sensitive detection relies on simple, basic principles: 1) The molecule-generated electric field in the forward direction (the ``darkness wave'') is most precious, and contains the maximum available information. It is the physical manifestation of the -alpha*L term appearing when Beer's absorption law is expanded for low opacity. 2) After generating the molecular signal, the detected unabsorbed light sets the ``shotnoise'' measurement floor. We use an optical transmission cavity for the gas cell, since the high internal ``build-up'' field will elicit a strong molecular ``darkness wave,'' for a given transmitted dc light on the detector. A more conventional explanation of Cavity Enhancement would be the extension of the effective cell length by the factor Finesse * 2/. 3) To avoid excess low-frequency noise, such small signals should be measured by ac methods, comparing on-resonant and off-resonant cases in quick succession. 4) By SIMULTANEOUSLY obtaining and subtracting these cases, one provides a signal channel with NO OUTPUT unless there is a resonance. This is conveniently accomplished by using the FM detection method with the optical heterodyne sidebands and carrier being transmitted through the cavity via adjacent axial orders. 5) A remarkable property of this configuration, when the modulation frequency = cavity mode spacing, is the suppression of ANY detection of laser frequency noise with the transmitted light. We can refer to this property as (laser FM-) Noise-Immune detection. This enables profitable use of cavity finesse in the range above 10,000, without a noise penalty. Altogether, we refer to this new spectroscopy as Noise-Immune, Cavity-Enhanced, Optical Heterodyne Molecular Spectroscopy, ie. ``NICE OHMS''. We have measured excellent saturated dispersion signals from HCCH in the band near 790 nm (₁ + 3 ₃), and recently, using a Nd:YAG source, from the HCCD P(5) line (₂ + 3 ₃) at 1064 nm. The saturation peaks broaden from the transit limit of 270 kHz near zero pressure at the rate 34.7 kHz/mTorr. We observe interesting narrow linewidths 8-fold below the transit limit, basically using optical selection of slow molecules by low power and pressure (~2 mW and 2 mTorr). The absolute frequency is measured to be 281,635,363.960 MHz \pm 45 kHz.