High-sensitivity CW Fabry-Perot enhanced spectroscopy of CO2 and C2H2 using a 1064-nm Nd : YAG laser

We have investigated the weak absorption lines of CO2 (20(0)3 <-- 00(0)0) R (6), C2H2 (2100(0)1(1) <-- 0000(0)0(0)) R(12), and C2H2 (1200(0)3(1) <-- 00 00(0)0(0)) P(6) near 1064 nm using CW cavity ring-down spectroscopy and CW cavity transmission. An effective interaction length of more than 30 km was achieved (cavity finesse approximate to 86000) and the sensitivity was app roximate to 10(-8) cm(-1). Higher sensitivity of 3.3 x 10(-11) cm(-1)/root Hz was obtained by cavity-enhanced wavelength modulation spectroscopy. The absorption coefficients, pressure broadening coefficients, lineshapes, line strengths, and absolute wavelengths of those lines were determined, some o f them for the first time. The measurements were carried out over a wide ra nge of pressures from the strongly inhomogeneous to the strongly homogeneou s region.