The near ultraviolet band system of singlet methylene

In a classic paper by G. Herzberg and J. W. C. Johns entitled "The Spectrum and Structure of Singlet CH2" (Proc. Roy. Sec. A 295, 107-128 (1966)) the analysis of the (b) over tilde B-1(1) <-- (a) over tilde (1)A(1) red absorp tion band system of CH2 is discussed in detail for the first time. In addit ion to that band system the observation of a fragment of a weak near ultrav iolet absorption band system is reported. The three observed bands of the s ystem could not be vibrationally assigned or rotationally analyzed but it w as pointed out that they probably involve absorption into the second excite d singlet state, (c) over tilde (1)A(1). We show this supposition to be tru e here by simulation. In order to simulate the spectrum we have calculated ab initio the (c) over tilde-(a) over tilde and (c) over tilde-(b) over til de transition moment surfaces and used the MORBID and RENNER program system s with previously determined potential energy surfaces for the (a) over til de, (b) over tilde, and (c) over tilde states in a calculation of the energ y levels and wavefunctions. We find that the three bands seen by Herzberg a nd Johns are part of the (c) over tilde <-- ((a) over tilde/(b) over tilde) system but that all of the bands of the system above about 31 000 cm(-1) a re missing as a result of (c) over tilde state predissociation. We vibratio nally assign the bands but the weakness of the spectrum, and the presence o f perturbations, make it impossible for us to analyze the rotational struct ure fully. Further experimental and theoretical studies are suggested. (C) 2001 Academic Press.