1. Zhong Wang, Trevor Sears Department of Chemistry, Brookhaven National Laboratory; Department of Chemistry, Stony Brook University Ju Xin Department of Physics and Engineering Technology Bloomsburg University

2. Methylene (CH 2 ): the simplest carbene Degrees Energy / cm -1 H H 1B11B1 3B13B1 H H 1A11A1 H H Two singlet states are degenerate at linearity. Transitions between the a(singlet) and b(singlet) states result in an electronic spectrum from near-IR through visible.

3. Excimer Laser Cell Diode Laser Detector HP AP EOM LO LP Digital Scope PC SABS IF RF Experimental Setup FM Transient Absorption Spectrometer CH 2 CO + Ar CH 2 CO + 308nm → CH 2 + CO EOM: electro-optic modulator; RF: radio frequency; IF: intermediate frequency; LO: local oscillator; LP: low-pass filter; HP: high-pass filter; BS: beam splitter; SA: spectral analyzer; AP: amplifier.

4. Herzberg and Johns previously reported only approximately 50 lines between 12550 cm -1 and 13000 cm -1. We have approximately 1000 lines in our data. A section of the frequency modulated laser absorption spectrum of CH 2 near 791 nm.

5. Vibronic levels assigned AssignedKaKa Observed b (cm -1 )Calculated a (cm -1 ) ã(0, 11, 0)11263512668 ã(0, 11, 0)312936 (1, 1, 0)01253512502 (1, 1, 0)11294312964 (0, 3, 0)11297212988 ã (2, 6, 0)11283412837 (0, 2, 0)41270812669 a From Ref. J.-P. Gu et al., J. Mol.Spectrosc. Vol. 517-518, (2000), 247-264. All frequencies are relative to the ã(0, 0, 0) 0 00 level. b Derived from fitting the assigned rotational structure in each band separately.

6. Some assigned rotational lines J’Ka’Ka’Kc’Kc’J’’K a ’’K c ’’ Upper vibrational level(cm -1 ) 111101ã(0, 11, 0) K a = 112637.155 11122112556.892 11022012553.303 11020212599.306 11000012652.975 21232212541.112 21222012594.802 21220212640.820 21132112529.232 21130312583.421 21122112589.220 21110112669.493 31342312527.036 31332112593.905 31330312648.120 31322112653.921 … … … 1 11 -1 01 2 12 -2 02 3 13 -3 03

7. YAG OPO AP Excimer Laser Cell Diode Laser Detector HP AP EOM LO LP Digital Scope PC SABS IF RF Mixer Detailed OODR setup FM Transient Absorption Spectrometer a b'b' b

8. a (0,11,0) – a (0,0,0) 1 11 -1 01 at 12637.18 cm -1, 2 12 -2 02 at 12640.85 cm -1, 3 13 -3 03 at 12648.08 cm -1. 1 11 -1 01 2 12 -2 02 3 13 -3 03 0.2 0.4 0.6 0.8 microsecs Probe laser: at 12648.08 cm -1 Pump laser: at 16058.28 cm -1

9. Molecular parameters for the upper levels (cm -1 ) AssignedKaKa BuBu DuDu ququ (0,11,0)19.413(32)0.0129(09)-0.989(30) (0,11,0)39.426(08)0.0260(02)0.000246(00) (1, 1, 0)07.736(35)0.0033(08)  (1, 1, 0)19.64(13)0.0650(52)0.30(16) (0, 3, 0)17.906(11)-0.0587(04)-0.574(12) (2, 6, 0)18.443(49)0.0156(13)-1.725(59) (0, 2, 0)48.088(27)0.0011(04)-0.000000(00) a One standard deviation limit in parentheses. b 167 lines were used in the fit.

10. Summary I.Rotational branches in 7 vibronic bands involving K a = 0−4 have been assigned using known ground state combination differences. Most of them have not previously been observed and some reassignments of the Herzberg and Johns analysis have been made. II.Comparison with the most complete available calculated ro- vibronic energy level structure helped considerably in making the assignments, and the observed vibronic levels are assigned to levels of both and electronic character. The calculated energy levels show moderate, up to 10 cm −1, apparently random, differences from the observed levels. III.The new data will certainly help to refine the singlet potential and also provide additional avenues for future kinetics and dynamics studies of the radical.

11. Work at Brookhaven National Laboratory was carried out under Contract No. DE- AC02-98CH10886 with the U.S. Department of Energy. Dr. Ju Xin was supported by the Faculty and Student Teams program of the Educational Programs Department at Brookhaven National Laboratory. Acknowledgment