Zhong Wang, Trevor SearsDepartment of Chemistry,Brookhaven National Laboratory;Department of Chemistry,Stony Brook University

Ju XinDepartment of Physics and Engineering TechnologyBloomsburg University

Methylene (CH2): the simplest carbene

0.0

-0.5

0.5

1.0

1.5

2.0

2.5

3.0x104

/degrees

CH2 bending potentialcurves

X (Triplet)

a (Singlet)

b(Singlet)

Degrees

En

erg

y /

cm-1

H

H1B1

3B1H

H

1A1

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.

ExcimerLaser

Cell

Diode Laser

Detector

HP

AP

AP

EOM

LO

LPDigitalScope

PC

SABS

IF

RF

Experimental Setup

FM Transient Absorption Spectrometer

CH2CO + ArCH2CO + 308nm → CH2 + 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.

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.

12550 12600 12650 12700 12750 12800 12850 12900 12950 13000

Wavenumber (cm-1)

12636 12638 12640 12642 12644 12646 12648

Wavenumber (cm-1)

A section of the frequency modulated laser absorption spectrum of CH2 near 791 nm.

Vibronic levels assigned

Assigned Ka Observedb(cm-1) Calculateda(cm-1)

ã(0, 11, 0) 1 12635 12668

ã(0, 11, 0) 3 12936 12936

(1, 1, 0) 0 12535 12502

(1, 1, 0) 1 12943 12964

(0, 3, 0) 1 12972 12988

ã (2, 6, 0) 1 12834 12837

(0, 2, 0) 4 12708 12669

b~

b~

b~

b~

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) 000 level.b Derived from fitting the assigned rotational structure in each band separately.

Some assigned rotational lines

J’ Ka’ Kc’ J’’ Ka’’ Kc’’ Upper vibrational level (cm-1)

1 1 1 1 0 1 ã(0, 11, 0) Ka = 1 12637.155

1 1 1 2 2 1 12556.8921 1 0 2 2 0 12553.3031 1 0 2 0 2 12599.3061 1 0 0 0 0 12652.9752 1 2 3 2 2 12541.1122 1 2 2 2 0 12594.8022 1 2 2 0 2 12640.8202 1 1 3 2 1 12529.2322 1 1 3 0 3 12583.4212 1 1 2 2 1 12589.2202 1 1 1 0 1 12669.4933 1 3 4 2 3 12527.0363 1 3 3 2 1 12593.9053 1 3 3 0 3 12648.1203 1 3 2 2 1 12653.921… … …

12636 12638 12640 12642 12644 12646 12648

Wavenumber (cm-1)

111-101 212-202 313-303

YAG

OPO

AP

ExcimerLaser

Cell

Diode Laser

Detector

HP

AP

EOM

LO

LPDigitalScope

PC

SABS

IFRF

Mixer

Detailed OODR setupFM Transient Absorption Spectrometer

a

b'

b

a (0,11,0) – a (0,0,0)

111-101 at 12637.18

cm-1,

212-202 at 12640.85

cm-1,

313-303 at 12648.08

cm-1.

12636 12638 12640 12642 12644 12646 12648

Wavenumber (cm-1)

111-101 212-202 313-303

0.2 0.4 0.6 0.8microsecs

Probe laser: at 12648.08 cm-1

Pump laser: at 16058.28 cm-1

ρ/degrees

En

erg

y /

cm-1

Pump laser

Probe laser

0.0

-0.5

0.5

1.0

1.5

2.0

2.5

3.0x104

CH2 bending potentialcurves

X(Triplet)

a(Singlet)

b(Singlet)

ρ/degrees

En

erg

y /

cm-1

Pump laser

Probe laser

0.0

-0.5

0.5

1.0

1.5

2.0

2.5

3.0x104

CH2 bending potentialcurves

X(Triplet)

a(Singlet)

b(Singlet)

Molecular parameters for the upper levels (cm-1)

Assigned Ka Bu Du qu

(0,11,0) 1 9.413(32) 0.0129(09) -0.989(30)

(0,11,0) 3 9.426(08) 0.0260(02) 0.000246(00)

(1, 1, 0) 0 7.736(35) 0.0033(08)

(1, 1, 0) 1 9.64(13) 0.0650(52) 0.30(16)

(0, 3, 0) 1 7.906(11) -0.0587(04) -0.574(12)

(2, 6, 0) 1 8.443(49) 0.0156(13) -1.725(59)

(0, 2, 0) 4 8.088(27) 0.0011(04) -0.000000(00)

a~

a~

a~

b~

b~

b~

b~

a One standard deviation limit in parentheses.b 167 lines were used in the fit.

Summary

I. Rotational branches in 7 vibronic bands involving Ka =

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.

a~ b~

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