Journal of Molecular Spectroscopy 267 (2011) 50–57

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Journal of Molecular Spectroscopy

journal homepage: www.elsevier .com/locate / jms

Transient laser absorption spectroscopy of CH2 near 780 nm

Chih-Hsuan Chang a, Zhong Wang b, Gregory. E. Hall a, Trevor J. Sears a,c,⇑, Ju Xin d

a Chemistry Department, Brookhaven National Laboratory, Upton, NY 11973, USAb Mathematics and Science Department, Suffolk County Community College, Riverhead, NY 11901, USAc Chemistry Department, Stony Brook University, Stony Brook, NY 11794, USAd Department of Physics, Bloomsburg University, Bloomsburg, PA 17815, USA

a r t i c l e i n f o

Article history:Available online 19 February 2011

Keywords:SpectraRotationally resolvedVibronicNear-infrared

0022-2852/$ - see front matter � 2011 Elsevier Inc. Adoi:10.1016/j.jms.2011.02.004

⇑ Corresponding author at: Chemistry Departmentratory, Upton, NY 11973, USA. Fax: +1 631 344 5815.

E-mail address: sears@bnl.gov (T.J. Sears).

a b s t r a c t

Bands in the CH2~b1B1—~a1A1 transition between 12500 and 13000 cm�1 were recorded at Doppler-lim-

ited resolution using a transient frequency-modulation (FM) laser absorption spectrometer. Rotationallevels in seven upper vibronic states: ~að0; 11; 0Þ1;3; ~að2; 6; 0Þ1, ~bð0; 2; 0Þ4; ~bð1; 1; 0Þ1;2, and ~bð0; 3; 0Þ1

were assigned with the assistance of optical–optical double resonance (OODR) and ground state combi-nation differences. Perturbations due to rotation-vibration coupling, anharmonic couplings, ‘-resonance,as well as the Renner–Teller effect are discussed as sources of the irregular rotational patterns observedin the ~bð0; 3; 0Þ1; ~bð1; 1; 0Þ1, and ~að0; 11; 0Þ3 vibronic levels.

� 2011 Elsevier Inc. All rights reserved.

1. Introduction

CH2, the simplest carbene, has two electrons to distribute in itsouter 3a1 and 1b1 symmetry valence molecular orbitals, giving riseto four low-energy states. The eX3B1 and ~b1B1 states correspond tothe triplet- and singlet-coupled, single-occupancy configurations,while the ~a1A1 and ~c1A1 states correspond primarily to configura-tions with double occupation of the in-plane 3a1 orbital or theout-plane 1b1 orbital, respectively. The ~a1A1 and ~b1B1 states corre-late with a 1Dg state at the linear geometry and coupling between~a1A1 and ~b1B1 states is a classic example of the vibronic Renner–Teller effect, which may be viewed either as a Coriolis interactionbetween two bent molecule states, or as an electrostatic splittingof the doubly degenerate linear molecule bending potential func-tion [1]. The measurement, calculation, and interpretation of therovibronic levels and electronic structure of methylene have beenan active area of research for more than 50 years. Both the sin-glet-triplet splitting (see review and references cited in [2]) andthe Renner–Teller perturbed rovibronic structure in the lowest sin-glet states [1,3–10] are classic problems in theoretical quantumchemistry.

Since the high resolution absorption spectrum of the singletstate molecule was first analyzed by Herzberg and Johns [11],experimental studies of the spectroscopy have provided ever moreprecise and complete energy levels to challenge the theoreticalmodeling. Studies include visible laser-induced fluorescence,

ll rights reserved.

, Brookhaven National Labo-

absorption and magnetic-rotation spectroscopy by the Mooregroup [12–15], stimulated emission pumping by Dai’s group[16,17], and extension of the spectra to the near-infrared usingtransient absorption spectroscopy [18–25]. Finally, the band originof the ~b1B1—~a1A1 system near 1.20 lm was observed and rotation-ally analyzed very recently [26]. The assignment and interpretationof the spectra over the years has been greatly assisted by calcula-tions of ab initio or empirically corrected ab initio potential energysurfaces. Green et al. [3], Duxbury et al. [4–7] and Bunker and Jen-sen et al. [8,9] have developed models to simulate the rovibroniclevel structure and the electronic spectrum including the effectsof rovibronic coupling between the ~a1A1 and ~b1B1 states. Recentwork by Gu et al. [9,27] fit ab initio potential energy surfaces forthe ~a1A1 and ~b1B1 states to analytical expressions, and calculatedtransition frequencies and intensities as well as nuclear dynamicson these surfaces. Their results have proven to be the most usefulin guiding spectral assignments in the ~b—~a system.

While much of the singlet absorption spectrum has now beeninvestigated using high resolution laser-based techniques, thereremained a section between the visible and near-infrared thathad not been studied since the initial work of Herzberg and Johns[11]. In this paper, we report the observation of the Doppler-lim-ited absorption spectrum between 12500 and 13000 cm�1, par-tially filling this gap We assign bands terminating in sevenexcited-state vibronic levels, most of which have not previouslybeen observed. Rovibronic assignments are difficult due to manylocal perturbations and many accidental coincidences between ob-served spectral line spacings and ground state combination differ-ences. In the present work, extensive optical–optical doubleresonance (OODR) measurements have been used to label knownlower state levels and confirm rotational assignments.

12600 12700 12800 12900 13000cm-1

Fig. 2. Stick diagram of assigned CH2 transitions in the region of 12500–13000 cm�1.

C.-H. Chang et al. / Journal of Molecular Spectroscopy 267 (2011) 50–57 51

2. Experimental

Transient frequency-modulation (FM) laser absorption spec-troscopy has been applied to study the spectroscopy of methyleneand other transient species in this laboratory during the past15 years. Details of the absorption spectrometer and experimentalresults have been published elsewhere. [20–22,28] In the presentexperiments, the probe laser was either an extended cavity diodelaser (Sacher Lasertechnik 520 series) or a Ti:sapphire ring laser(Coherent 899 with Verdi pump). The laser beam was frequencymodulated (Model 4003 electro-optic modulator, New Focus) at191 MHz, passed through a 1 meter long absorption cell, and im-aged onto a Si photodiode (Hamamatsu S3883). The photodiodeoutput was separated by a bias-T into an ac signal componentand a dc level for power monitoring. The amplified ac componentwas demodulated in a double-balanced mixer to give an in-phasetransient absorption signal, which was amplified, lowpass filtered(Mini Circuits, DC-23 MHz), digitized and averaged in a digitaloscilloscope (LeCroy Waverunner, LT354) and archived for furtheranalysis. Wavelength calibration was performed with a commer-cial wavemeter (Burleigh, WA1500).

A V-type OODR scheme was used to confirm rotational assign-ments. The laser pulse from an optical parametric oscillator system(Spectra-Physics MOPO-HF, pumped by a Spectra-Physics modelPRO-290) was used to bleach the population of CH2 from a~a1A1ð0; 0; 0Þ rotational level to a higher vibrational level in the~b1B1 state using known visible transitions [12,13]. When in reso-nance with a transition involving a common lower level, the bleachlaser generates the dip signal in the transient absorption wave-form. The wavenumber of the bleach laser was measured with aCoherent Wavemaster wavemeter. The entire spectrometer anddata acquisition was controlled by a LabView program via a GPIBIEEE-488 bus.

12656 12660

514-422

a(0,11,0)

615-52

a(0,11,

441-331

440-330

414-404

a(0,11,0)

cm

717-827

a(2,6,0)

Fig. 1. Section of the raw frequency-modulation spectrum of the rR3J�2 and rR3J�3 branrotational assignments.

Singlet CH2 was formed in the 308 nm photolysis of ketene(CH2CO) which was itself made by dehydration of acetic anhydride,(CH3CO)2O, carried in a slow flow of helium past a heated filament.The photolysis beam from a XeCl excimer laser (Lambda Physik,LPX200, 15–20 mJ/pulse) was combined and separated with thebleach and probe laser beams along the axis of the gas flow cellwith 308 nm dichroic mirrors. The flow cell system was main-tained at 1 Torr, evacuated by a liquid nitrogen-trapped mechani-cal pump.

12664 12668

643-533

542-432

541-431642-532

3

0)

-1

ches of the CH2~bð0; 2; 0ÞK¼4—~að0; 0; 0Þ transition in the 12660 cm�1 region, with

Table 1Observed rovibronic transitions originating from ~a1A1ð0; 0; 0Þ CH2 in the region of 12500–13 000 cm�1.

J0Ka0Kc0a J00Ka00Kc00

b –f ~m Intensityc J0Ka0Kc0a J00Ka00Kc00

b f ~m Intensityc

~b(0,3,0)1d ~a(0,11,0)1d

111 221 12889.867 18 111 221 12556.892 1012988.3984

e 101⁄ 12970.126 17 12655.4252 101

⁄ 12637.155 40110 220

⁄ 12887.787 8 110 220 12553.303 412987.43220 202

⁄ 12933.792 1 12652.94924 202 12599.306 0.7000

⁄ 12987.452 5.5 000⁄ 12652.975 7

212 322⁄ 12869.619 8 212 322 12541.112 7

13022.9648 220⁄ 12923.329 2 12694.4465 220 12594.802 0.9

202 12969.340 12.5 202⁄ 12640.820 16

211 321⁄ 12861.191 26 211 321 12529.232 23

13019.7206 303⁄ 12915.388 4 12687.7589 303 12583.421 0.8

221⁄ 12921.188 15 221 12589.220 3.8

101⁄ 13001.447 36 101

⁄ 12669.493 32313 441 12722.069 1.5 313 423 12527.063 2713078.03830 423 12852.637 16 12752.4518 321 12593.905 1.8

321⁄ 12919.451 7 303

⁄ 12648.120 60303 12973.719 62 221

⁄ 12653.921 4.8221 12979.518 4 312 422 12501.782 7.5

312 440 12716.206 1 12740.48917 404 12571.525 0.413072.20514 422

⁄ 12833.491 6 322 12587.146 0.9404

⁄ 12903.238 0.5 220 12640.820 16322 12918.868 2 202

⁄ 12686.875 6220

⁄ 12972.566 4 414 524 12514.625 10414 542 12700.625 1.7 12828.5831 404

⁄ 12659.619 7013150.2343 524 12836.273 2 413 523 12471.476 1

404⁄ 12981.269 9.5 12811.13226 423 12585.709 4

322 12996.900 1 321 12652.585 7.5413

h 541 12673.561 1.8 303⁄ 12706.804 7

13123.50123 523 12783.816 7 515 625 12503.969 27441 12767.522 0.5 12922.2683 505

⁄ 12674.907 96423

⁄ 12898.085 22 423 12696.875 10321

⁄ 12964.958 4.5 514 422⁄ 12661.523 1.2

413h 541 12697.223 5 12900.2557

13147.1399 523⁄ 12807.437 3.5 616 606

⁄ 12692.564 66441 12791.151 1 13032.1027

321⁄ 12988.608 7.5 615

i 707 12556.765 2515 643 12642.786 5. 13002.42534 625 12584.137 0.813206.7828 625 12788.489 2. 523 12662.763 7

541 12756.873 0.5 505⁄ 12755.074 1

523 12867.085 3. 717 707⁄ 12711.847 50

505⁄ 12959.413 25. 13157.47626 625 12739.197 6

514h 642 12633.376 6.7

13198.42415 624 12738.706 6.606 12858.898 2524

⁄ 12884.451 7.440 12842.424 1.422

⁄ 12959.690 2.616

h 726⁄ 12860.065 6

13397.6109 642 12832.539 2.5624

⁄ 12937.879 7615

h 743 12589.220 3.813289.25618 725

⁄ 12692.240 106.707 12843.620 1.625 12870.955 29.523 12949.583 18.

717h 827

⁄ 12771.878 113443.41527 743 12743.358 1.8

707⁄ 12997.811 13.5

~b(0,2,0)4d ~a(2,6,0)1d

441 431g 12585.167 3 111 101

⁄ 12833.149 5.512869.55711 431

g . . . . . . 12851.4215 221 12752.890 4413 12659.434 1.5 110 220

⁄ 12748.044 1331

⁄ 12661.008 44 12847.71334 202 12794.118 8313 12762.045 0.5 212 322 12735.300 2

440 432 12586.058 10 12888.6402 202⁄ 12835.022 8

12869.5295 414 12699.085 1.8 211 321 12719.210 3330 12660.771 80 12877.72613 303 12773.393 1312 12737.547 0.5 221 12779.187 2

542 616 12610.432 4 101⁄ 12859.448 6

12950.1339 532 12568.448 13 313 423 12718.497 2514 12645.715 18 12943.89210 303

⁄ 12839.554 10432 12666.668 65 221 12845.366 2414 12779.679 1 312 422

⁄ 12683.718 10

52 C.-H. Chang et al. / Journal of Molecular Spectroscopy 267 (2011) 50–57

Table 1 (continued)

J0Ka0Kc0a J00Ka00Kc00

b –f ~m Intensityc J0Ka0Kc0a J00Ka00Kc00

b f ~m Intensityc

541 533 12573.130 7 12922.43413 322 12769.086 112949.94315 515 12702.031 1 202

⁄ 12868.809 2431

g ⁄ 12665.554 60 414 524⁄ 12702.323 2.7

431g 12662.763 7 13016.2774 404

⁄ 12847.310 14413 12739.842 1 322 12862.937 0.7

643 633 12545.689 4 413 523 12641.945 2013046.30114 615 12633.376 6.7 12981.63211 423 12756.223 1

551 12503.763 1 321⁄ 12823.109 5

533 12669.493 32 303⁄ 12877.297 3

515 12798.395 0.7 515 643 12541.112 7642 634 12557.337 23 13105.09315 625 12686.799 2213045.49110 616 12705.769 5 541 12655.173 0.7

550 12502.947 4 505⁄ 12857.721 30

532 12663.809 62 441 12749.082 .3514

⁄ 12741.060 3 423⁄ 12879.702 2

514 606 12715.744 113055.30523 542 12605.669 1

524 12741.428 0.8440 12699.343 5422 12816.598 2404

⁄ 12886.354 1616 726 12671.714 213209.2787 606

⁄ 12869.742 14524

⁄ 12895.326 2615

i 725 12544.549 4.513141.58218 707 12696.001 1

625 12723.283 3523 12801.886 4505

⁄ 12894.200 2717 827 12656.934 2213328.49715 707

⁄ 12882.872 37625

⁄ 12910.216 1.5

~b(1,1,0)1d ~a(0,11,0)3

111 101⁄ 12921.188 15 331

12939.4577 . . .j

111 101⁄ 12922.733 1 330

12941.0027 . . .j

110 432

. . .j . . .j

212 431 523⁄ 12755.074 1

. . .j 13094.76332 441 12738.808 1211 321 12832.777 4 423

⁄ 12869.353 212991.28425 221

⁄ 12892.735 1.2 303 12990.402 1101

⁄ 12973.000 1.8 431 541 12699.458 8.5313 441

⁄ 12701.963 6 13149.37016 523⁄ 12809.684 10

13057.94411 423 12832.539 3 441 12793.398 1321 12899.408 4 423 12923.953 3303

⁄ 12953.606 12 321⁄ 12990.835 8.5

221 ⁄ 12959.413 25 533

312 440 12699.343 5 . . .j

13055.32212 422 12816.598 2 532 624⁄ 12728.394 2

404 12886.354 1 13188.11612 542 12738.492 2322

⁄ 12901.999 7 524⁄ 12874.166 2

202⁄ 13001.685 4

414

. . .j

413 541 12642.365 113092.25315 523 12752.531 1

303⁄ 12987.934 19

515 625⁄ 12771.616 1

13189.91623 523 12850.255 1505

⁄ 12942.555 9514

. . .j

616 726 12795.371 1.13332.90828 606

⁄ 12993.350 6.616 726 12771.878 113309.4097 606

⁄ 12969.871 5615 725 12667.208 2.113264.28934 643

⁄ 12700.314 11625

⁄ 12845.992 4523 12924.613 2

717 827⁄ 12742.685 2

13414.243 707⁄ 12968.610 10

(continued on next page)

C.-H. Chang et al. / Journal of Molecular Spectroscopy 267 (2011) 50–57 53

Table 1 (continued)

J0Ka0Kc0a J00Ka00Kc00

b –f ~m Intensityc J0Ka0Kc0a J00Ka00Kc00

b f ~m Intensityc

717 827⁄ 12760.830 1

13432.389 707⁄ 12986.775 10

~b(1,1,0)2d ~a(1,9,0)2

221 331⁄ 12959.974 0.5 726 716

g ⁄ 12715.744 113168.5407 13248.31927 716

g 12709.377 2220 652

⁄ 12593.115 1.8. . .j 634

⁄ 12760.180 9322 432

⁄ 12934.642 1113218.1097

321

. . .j

423 533⁄ 12906.411 5

13283.2237

422 532⁄ 12901.999 7

13283.6867

524 634⁄ 12874.893 4

13363.07824 652 12707.894 3

a Upper state rotational quantum number.b Rotational quantum number for the lower state involved in the transition.c Observed wavenumber (cm�1) with estimated error limit of 0.007 cm�1and measured intensity (arbitrary units).d Vibronic level identification based on the results of reference [9].e The term value energy relative to the 000 level of ~a1A1ð0; 0; 0Þ, derived from the measured transition and known ground state rotational combination differences.

Subscripts are the standard deviation of the multiple individual determinations of the energy in units of the last quoted significant figure.f Transitions confirmed by double resonance experiments are marked by (⁄).g Triplet perturbation splittings are observed for the 431, 716, and 818 ground state levels.h These perturbed levels in the upper state are not included in the rotational fitting.i The transition is not included in the rotational fitting due to severe perturbation.j The transitions terminating in this upper level could not be identified.

16577.5 16578.0 16578.5 16579.00

10

20

30

40

Dep

letio

n si

gnal

(%)

cm-1

Fig. 3. Scan of the bleach laser across the 322–432 transition in the~bð0; 15; 0Þ—~að0; 0; 0Þ band at 16578.362 cm�1, with the diode laser tuned to atransition at 12934.642 cm�1. The observed depletion supports the assignment ofthe diode laser transition to a 322–432 transition in the ~bð1; 1; 0Þ2—~að0; 0; 0Þ3 band.

54 C.-H. Chang et al. / Journal of Molecular Spectroscopy 267 (2011) 50–57

3. Results and discussion

In the spectral region between 12500 and 13000 cm�1, Herz-berg and Johns [11] only partially assigned three vibronic bands:~að0; 11; 0Þ1—~að0; 0; 0Þ0, ~bð0; 3; 0Þ1—~að0; 0; 0Þ0, and ~bð0; 2; 0Þ4—~að0; 0; 0Þ3;5. All recent calculations agree that these transitionsshould be the stronger ones in this region. However, calculationsalso predict that the ~að2; 6; 0Þ1, ~bð1; 1; 0Þ0;1;2; ~að1; 9; 0Þ2 and~að0; 11; 0Þ3 levels should be observable in this region [9]. In thenew spectrum, we have observed 822 lines; the complete list isavailable as Supplementary information from the journal. Fig. 1shows a section of the rR3J�2 and rR3J�3 branches of the~bð0; 2; 0Þ4—~að0; 0; 0Þ transition of CH2 near 12660 cm�1, originallyassigned by Herzberg and Johns. Other transitions, not reported inthe original work, are present. These form part of a typically irreg-ular pattern of absorption lines in the spectrum, which makes se-cure rotational assignments difficult. The vibronic assignmentsshown in the figure were guided by the results of the theoreticalcalculation and rotational assignments confirmed by OODR.

Fig. 2 is a stick spectrum showing all the assigned transitions inthe region studied and the detailed assignments are summarized inTable 1 together with upper state term values. About 28 % of theobserved transitions in this region are assigned. This may seem dis-appointingly low, but in fact represents a considerably higher frac-tion of assigned lines than in any other parts of the ~b—~a system,even near the origin at longer wavelengths [26], where the densityof background levels is much smaller. The extensive OODR workhas enabled this success. Fig. 3 shows a typical depletion spectrumas the OPO scans across the 322–432 transition of the ~bð0; 5; 0Þ—~að0; 0; 0Þ band at 16578.362 cm�1 [12], while the diode laser is res-onant with the 322–432 transition of the ~bð1; 1; 0Þ—~að0; 0; 0Þ band.The peak depletion is about 27% of the absorption signal. The ob-served line width (0.36 cm�1) of the bleaching resonance is broad-er than either the CH2 Doppler width (0.04 cm�1) or the pulsedlaser linewidth (0.07 cm�1), and is likely dominated by powerbroadening. The assignments for each vibronic level can also be

cross-checked on a different rotational branch in a different OODRscheme.

In this work, all the upper state levels lie well above the barrierto linearity of the Renner–Teller states, and they should be ade-quately described by a pseudo-linear molecule Hamiltonian foreach vibronic K stack (Ka in the bent molecule limit) as:

FðJ;KÞ ¼ m0 þ BmJðJ þ 1Þ � DmJ2ðJ þ 1Þ2 þ HmJ3ðJ þ 1Þ3 � 1=2½qJKðJ

þ 1ÞK � qdJ2KðJ þ 1Þ2K �

where m0, Bm, Dm, and q are the vibronic band origin, effective rota-tional constant, effective centrifugal distortion constants, and K-type doubling parameters for each Ka stack, respectively. Hm and

Table 2Fitted molecular parameters for the assigned levels (in cm�1).

~b(0,2,0)4 ~b(1,1,0)2 ~b(0,3,0)1c ~a(0,11,0)1 ~a(2,6,0)1

Bm 8.1885(28)a 8.4155(49) 8.625(81) 9.2799(74) 8.4738(37)Dm � 102 0.2782(45) 0.8031(47) �0.77(39) 0.377(35) 0.871(16)Hm � 104 0.257(45) 0.430(20)qb �0.000000258(26) 0.001150(56) 0.476(29) 1.1791(53) 1.8784(26)qd � 102 1.498(22) 0.7345(94)m0 12706.688(94) 13118.358(38) 12970.09(37) 12635.591(39) 12832.652(15)r 0.012d 0.010 0.561 0.066 0.035

a One standard deviation of the fit in parentheses.b Energies of the Ka = 1, K-doublet levels are ±q J(J + 1), with upper sign for levels with Kc = J.c Results from fitting of the low J level energies only; highly perturbed 413, 515, 514, 616, 615, and 717 were not included.d Overall standard deviation of the fitted rotational levels for this vibronic level.

Table 3A comparison of the theoretical calculation and experimentally determination energy of CH2 in the region of 12500–13 000 cm�1.

K State ðm1; mbent2 ; m3Þ Experimenta Herzberg and Johns [11] Duxbury et al. [4]b Green et al. [3]b Gu et al. [9]

ðm1; mlinear2 ; m3Þ

1 ~a1A1 (0,11,0) 12652.94 ? 12647 12701 [~b(0,3,0)] 12672 [59% ~a(0,10,0) + 41% ~b(0,3,0)] 12666.57

1 (2,6,0) 12847.71 13071 13015 [67% ~a(2,6,0) + 33% ~b(1,5,0)] 12831.07

3 (0,11,0) � � � 13079 13111 [76% ~a(2,6,0) + 24% ~b(0,13,0)] 13029.31

1 ~b1B1(0,3,0) 12987.43 (0,9,0) 12987.4 12993 [~a(0,11,0)] 13008 [54% ~a(1,8,0) + 46% ~b(0,11,0)] 12986.41

1 (1,1,0) 12939.45 12866 12875 [63% ~a(1,8,0) + 37% ~b(1,5,0)] 12957.12

2 (1,1,0) 13168.54 13137 13175 [63% ~a(1,8,0) + 37% ~b(1,5,0)] 13189.60

4 (0,2,0) 12869.52 (0,10,0) 12869.5 12856 12873 [22% ~a(1,6,0) + 78% ~b(0, 12,0) ] 12877.62

a All energies are relative to that of the lowest rotational level of the ~a1A1 state.b Vibronic assignment shown when different from reference [9].

C.-H. Chang et al. / Journal of Molecular Spectroscopy 267 (2011) 50–57 55

qd are high-order correction terms. Here, we define the + and �signs to correspond to the Kc = J and Kc = J � 1 components respec-tively. The K-type doubling is most pronounced in the Ka = 1 levels,but is still measurable for the higher K values (Ka = 2, 3, 4 . . .). Theparameters derived from the fits to the ~bð0; 4; 0Þ4; ~bð1; 1; 0Þ2,~að0; 11; 0Þ1 and ~að2; 6; 0Þ1 vibrational levels are shown in Table 2with their standard deviations. Some strong localized and more sys-tematic vibronic level perturbations were noted and they are dis-cussed below. In fact, the ~að0; 11; 0Þ3; ~bð0; 3; 0Þ1, and ~bð1; 1; 0Þ1

vibronic levels appear to show significant mutual perturbations,and this makes the final fitting results problematical But, thereare no strong localized perturbations at low J and this implies thatfitting the rotational energies to extract the spectroscopic parame-ters is worthwhile and it facilitates comparison the published theo-retical results, see below. For the ~bð1; 1; 0Þ2 level, there are only fewtransitions of the pP3 branch expected in the region we study, asshown in Table 1. These transitions were also confirmed by OODR.

For most of the vibronic levels summarized in Table 2, the posi-tive effective centrifugal distortion constants (Dm) indicate thatthere is little or no overall ‘-type resonance rotation–vibrationinteraction. However, the negative value (�0.0077) found for the

Table 4A comparison of the theoretical and experimental determination of B and q for the assign

K State Level Bam Duxbury et al. [4] Gu

1 ~a1A1 (0,11,0) 9.2799 9.03 9.03

1 (2,6,0) 8.4738 8.513 (0,11,0) 7.981 ~b1B1

(0,3,0) 8.625 8.55 8.12

1 (1,1,0) 9.762 (1,1,0) 8.4155 8.784 (0,2,0) 8.1885

a Present work.b Determined from the fitting of the calculated levels to the pseudo-linear rotational

effective centrifugal distortion constant of the ~bð0; 3; 0Þ1 vibroniclevel indicates that it probably suffers from this type of interaction.The K-type doubling parameters for K = 4 and 2 levels shownin Table 1 are smaller (�0.000000258 and 0.001150 cm�1) than1.1791 and 1.8784 cm�1 in the K = 1 components of ~að2; 6; 0Þ1

and ~að0; 11; 0Þ1, as expected.

4. Comparison to calculations

The comparison of the vibronic level energies from the presentresults to some of the published calculations and the experimentalresults from Herzberg and Johns are shown in Table 3. The use oflinear ðmlinear

2 Þ or bent ðmbent2 Þmolecule bending vibrational quantum

numbers combined with Renner–Teller mixing of vibronic levelsmeans the labeling of some of these levels in the ~a and ~b statesis quite inconsistent in the literature. We have followed thescheme of Gu et al. [9] appropriate for a bent molecule. For exam-ple, the ~að0; 11; 0Þ1 vibronic level, 12659.94 cm�1 in this study, isidentified as ~bð0; 3; 0Þ1 level by Duxbury et al. [4], and predomi-nantly the ~að0; 10; 0Þ1 level by Green et al. [3] Duxbury et al. and

ed vibrational levels of CH2 in the region of 12500–13 000 cm�1

et al. [9]b qa Duxbury et al. [4] Gu et al. [9]b

1.1791 �1.83 2.03

1.8784 2.170.0007

0.476 �0.86 0.69

1.930.001150 0.0021�0.000000258

Hamiltonian model.

56 C.-H. Chang et al. / Journal of Molecular Spectroscopy 267 (2011) 50–57

Green et al. predict the position to be 12701 and 12672 cm�1,respectively. Overall, our experimental results show differencesof within 20 cm�1 compared to Gu et al. [9].

The effective rotational constants and K-type doubling parame-ters are summarized in Table 4 with comparisons to previousdeterminations. It has been shown [4] that the magnitude of theK-doubling (q) parameter carries information on the dominant

0 10 2012600

12800

13000

13200

13400

b(1,1,0)2

a(0,11,0)3

b(1,1,0)1

b(0,3,0)1

b(0,2,0)4

Term

Val

ue E

nerg

y (c

m-1

)

J

a(0,11,0)1

Fig. 4. Rotational term values of the assigned vibrational levels in the region

13000

13100

13200

13300

13400

13500

13600

110A2

211B2

312A2

413B2

514A2

615B2

716A2

111B2

212A2

313B2

414A2

515B2

616A2

717B2

717B2

110A2

211B2

312A2

413B2

514A2

615B2

716A2

cm-1

b(1,1,0)1

Fig. 5. Calculated [9] rotational energy levels and their rovibronic symmetry for thexperimental values, from this work, are plotted to the right.

contribution to the vibronic wavefunction. The results summarizedin Table 4 support the assignments. The sign of q depends on thelabeling scheme, and is opposite in the ~b1B1 and ~a1A1 states be-cause in the C2v group, the parity of the electronic function changesbetween these representations. The rotational term values fromRef. [9] were used to derive the effective constant by fitting energylevels to the pseudo-linear Hamiltonian as described above. The

30 40 50 60

a(1,9,0)2

J1J

a(2,6,0)1

(J+1)

J1J-1

of 12500–13000 cm�1. Open and closed symbols depict the K-doublets.

111B2

212A2

313B2

414A2

515B2

616A2

717B2

331A2

331A2432B2

432B2

533A2

533A2

330B2

330B2

431A2

431A2

532B2

532B2

633A2

a(0,11,0)3b(0,3,0)1

e ~bð1; 1; 0Þ1; ~bð0; 3; 0Þ1 and ~að0; 11; 0Þ3 vibronic levels. Available corresponding

C.-H. Chang et al. / Journal of Molecular Spectroscopy 267 (2011) 50–57 57

rotational levels for the ~bð0; 2; 0Þ4 were found to be perturbed inthe calculation, so no attempt was made to estimate the relativespectroscopic constants from the published energies. The experi-mental rotational constants determined from this work agree rea-sonably well with the theoretical calculation of Gu et al. However,the calculated K-type doubling constants are generally larger thanthe experimentally determined values. The ~bð0; 2; 0Þ4 rovibroniclevels have been experimentally studied previously [21,25] andthe results agree well with each other.

5. Perturbations

The rotational term values for several vibrational levels de-tected in this region are shown in Fig. 4 for clarity. Except for a veryfew missing levels, the assignment of the rotational levels of the~að0; 11; 0Þ1; ~bð0; 3; 0Þ1, ~bð0; 2; 0Þ4; ~að2; 6; 0Þ1 and ~bð1; 1; 0Þ2 vib-ronic levels is almost complete. The 615 rotational levels of both~að2; 6; 0Þ1 and ~að0; 11; 0Þ1 are found to be perturbed significantly.If DEdep � Eexp � Ecal, where Eexp is the term value for each rota-tional level as shown in Table 1, and Ecal is the calculated energyderived from the fitting parameters, then we found the DEdep forthe 615 rotational levels of ~að2; 6; 0Þ1 and ~að0; 11; 0Þ1 are �1.81and �6.62 cm�1, respectively. These two perturbed levels are as-cribed to accidental local perturbations and were not included inthe rotational fitting process. We found that the rotational levelsfor the ~að0; 11; 0Þ3; ~bð0; 3; 0Þ1, and ~bð1; 1; 0Þ1 vibronic levels lieclose in energy and mutual perturbations cause level shifts andmixing. In the ~bð0; 3; 0Þ1 vibronic level, we noted strong perturba-tions of the 515 and 616 rotational levels, causing large energyshifts. The 413 level is found to be split into two observed compo-nents, with an energy separation of 23.64 cm�1. The observation ofthis splitting is not too surprising since Petek et al. [12] observedsimilar phenomena in the ~bð0; 5; 0Þ1; ~bð0; 6; 0Þ0, and ~bð0; 6; 0Þ1

vibronic levels, with an energy splitting ranging from 5 to90 cm�1. The perturbation trend we observed experimentally forthe ~bð0; 3; 0Þ1 level is consistent with the theoretical calculationby Gu et al. [9], except for doublet splitting in the 413 level. Wenote that the high J assignments reported by Herzberg and Johnsfor this vibronic level are incorrect.

For the ~bð1; 1; 0Þ1 vibrational levels, we found that the rota-tional levels for 111 and 616 and 717 rotational levels also split intotwo components. The 431 rotational level in the ~að0; 11; 0Þ3 is alsoobserved and split into two components, in agreement with theprediction of Gu et al. [9] The predicted [9] and observed rotationallevels(JKaKc) for the three ~að0; 11; 0Þ3; ~bð0; 3; 0Þ1, and ~bð1; 1; 0Þ1

vibronic levels are shown in Fig. 5 with their rovibronic symmetry(Crev). It is challenging (and questionable) to assign the unambigu-ous rotational levels to a specific experimentally observed vibroniclevel. In many cases either the ~bð0; 3; 0Þ1 or ~bð1; 1; 0Þ1 assignmentcould be possible. As pointed out by Herzberg and Johns [11], andDuxbury et al. [4], vibrational (Fermi-) resonances occur betweenðm1; mbnet

2 ; 0Þ and ðm1 þ 1; mbnet2 � 2; 0Þ vibrational levels and will

complicate the spectral patterns. Furthermore, the excitation ofnon-zero K component will introduce the Renner–Teller couplingbetween the vibronic levels of ~a1A1 and ~b1B1 states. The rotation-vibration interaction between the ~bð0; 3; 0Þ1 and ~að0; 11; 0Þ3 vib-ronic levels may also play a role in shifting and mixing the rota-tional levels.

6. Conclusions

Using transient frequency-modulation spectroscopy, we haverecorded new spectra of CH2 in the 12500–13000 cm�1 region of

the ~b1B1—~a1A1 system. Rotational levels in the upper~að0; 11; 0Þ1;3; ~að2; 6; 0Þ1, bð0; 2; 0Þ4; bð1; 1; 0Þ1;2, and b(0,3,0)1 vib-ronic states have been assigned and the assignments confirmedusing a combination of OODR and ground state combination differ-ences. The band origins, effective rotational and K-type doublingconstants were derived assuming a pseudo-linear rotational Ham-iltonian model and compared to the recent calculations. An unex-pected and particularly complicated rovibronic structure wasobserved for b(1,1,0)1 and b(0,3,0)1 vibronic levels. Local perturba-tions from vibrational resonances, as well as Renner–Teller effectare responsible for these observations.

Acknowledgments

This work was carried out at Brookhaven National Laboratoryunder Contract No. DE-AC02-98CH10886 with the US Departmentof Energy, Office of Science, and supported by its Division of Chem-ical Sciences, Geosciences, and Biosciences within the Office of Ba-sic Energy Sciences. J. Xin thanks the Educational Programs Officeat Brookhaven National Laboratory for support from a Facultyand Teams summer program.

Appendix A. Supplementary data

A list of observed CH2 transition frequencies between 12500and 13000 cm�1 and their relative intensities is provided as sup-plemental data and is available from the Journal. Supplementarydata associated with this article can be found, in the online version,at doi:10.1016/j.jms.2011.02.004.

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