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

Abstract

A complete set of 27 normal mode vibrational constants ω⁰_i and x_i j as well as six harmonized vibrational frequencies ω_i 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 ω⁰_i and x_i 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.