Stimulated emission spectroscopy: A complete set of vibrational constants for X̃ 1A1 formaldehyde

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Abstract
A complete set of 27 normal mode vibrational constants ω0i and xi j as well as six harmonized vibrational frequencies ωi is obtained for the H2CO X̃1A1 state: ω01 =2811.42(15), ω02 =1755.858(40), ω03 =1500.32(49), ω04 =1170.224(30), ω05 =2861.30(14), ω06 =1250.565(63) x11=−28.95(14), x12=1.15(19), x13=−23.03(14), x14=−10.099(65), x15=−193.32(24), x16=−49.78(33) x22=−9.926(23), x23=−8.26(11), x24=−7.199(39), x25=−17.23(23), x26=6.581(49), x33=−0.164(97), x34=−1.769(52), x35=6.00(37), x36=−29.861(88), x44=−3.157(12), x45=−13.35(17), x46=−2.860(70) x55=−17.97(13), x56=−17.63(33), x66=−1.567(56), ω1=2977.91(31), ω2=1778.26(16), ω3=1528.95(54), ω4=1191.02(11), ω5=2997.04(36), ω6=1298.91(26) (1σ uncertainty in parentheses). These vibrational constants are derived primarily from stimulated emission pumping (SEP) spectra of more than 50 4500–9300 cm−1 vibrational levels of the X̃1A1 state, supplemented by partial rotational analyses of 12 4000–8100 cm−1 FTIR overtone and combination bands. This is the first time that the SEP technique has been systematically applied to a traditional but seldom achievable objective of high resolution vibrational spectroscopy, determination of a complete set of ω0i and xi j constants. Insofar as the rotationless vibrational levels of H2CO X̃1A1 can each be unambiguously assigned a set of normal mode quantum numbers and reproduced by a minimal set of vibrational constants, the X̃ state of formaldehyde remains vibrationally well organized up to at least 9300 cm−1.