Volume 121. number 4,s CHEMICAL PHYSICS LETTERS 15 November ?985

MULTIPHOTON SPECl-FtOSCOPY AND ANALYSIS OFT’HEEZII+X’II, BANDOFCH

Peter CHEN; William A. CHUPKA and Steven D. COLSON

Swrling Chemisrry Lnboraroq, Yale Uniuersily, New Haven, CT 06Sl I, USA

Received 19 July 1985

Rotalional analysis of a 2+i MPI specuum of CH radical. produced by UV mullipholon dissociation in a supersonic expansion. allows assignment of Ihe E srale to the u = 2 level of the D ‘n, slate. Populalion is observed in rotalional levels up 10 N =12.

1. Introduction

Theoretical treatments of electronic states and their interactions have been intimately connected with the molecular spectroscopy of diatomic and small poly-

atomic radicals. States cfneutral CH between 50000

cm-l and the ionization threshold are of particular

relevance to the dynamics of dissociative recombina- tion of CH+_ High interstellar concentrations of the

ion can be rationalized if calculations by Giusti-Suzor

and Lefebvre-Brian [l] concerning the 2pu2pn3so Rydberg potential curves can be supported by obser- vations of spectroscopic states perturbed by a 211 dis-

sociative curve. In the course of investigating laser photochemistry in a supersonic expansion, we have observed a spectrum of CH whose subsequent analysis

has resolved an anomaly in earlier work by Herzberg and Johns [2], and may serve as a first step towards experimental verification of the calculated potential curves.

In ref. [2], VUV absorption spectra were reported for an nd Rydberg series, and transitions terminating in three other states, with origins at 5898 1.0, 64211.7,and 64531.5 cm-l,designated D2ni, E 211,, and F ?Z+, respectively. The groundstate configura- tion is 2pu22pn. Three valente states arising from the 2pu2pn2 confguration lie near 30000 cm-l. Based on the sign and magnitude of its spin-orbit coupling constant, D 211i has been assigned a configuration of 2p7r3. F 2X+ was assigr+to.the 2pa23pa Rydberg state based on its position relative to the 3d Rydbergs.

0 009-2614/85/S 03.30 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division) ’

The 2pu23psr Rydberg state was predicted to lie at higher energy, with 2pu23s far to the red. The E state, therefore, was left unassigned. In addition, it was noted that no corresponding band could be found in CD. Herzberg’s observation of homogeneous broaden-

ing in the D and E states, and heterogeneous broaden-

ing in the F state, makes identification of the E state of interest_

2. Experimental

Spectra were recorded by two-photon resonant,

three-photon ionization in a home-built timeaf-flight mass spectrometer with ionization occurring in a pulsed free jet from which ions were extracted elec-

trostatically_ The doubled output (1.0 mJ, 8 ns) of a YAG-pumped dye laser (a Quanta-ray DCR-2A, PDL- 1, WEX-1 system) was focused with a 6” lens, and served to both photofragment an organic precursor, and ionize the fragments_ Resolution was roughly 1 cm-l. The particular mechanism for the photofrag- mentation will be discussed in a separate publication. Ketene was prepared by the flash vacuum pyrolysis of acetic anhydride, and purified by multiple trap-to- trap distillations; t-butylnitrite was prepared by the

action of acidic NaNO, on t-butanol and purified by distillation under reduced pressure_ Purity was check- ed by gas chromatography and mass spectroscopy. The ion. signal for m/e = 13 was digitized by a Tektronix 7912AD fast transient digitizer and record-

405

Volume 121, number 4,5 CHEMICAL PHYSICS.LETTERS .. 15 November 1985

ed as a function of laser wavelength. The signal was

averaged over 50 shots for each wavelength interval. The wavelength was calibrated with the atomic car- bon 3p lS,, + 2p lD2 resonance that appeared in two photons in the same spectral region.

3. Results and discussion

The CH E + X band is shown in fig. I_ Assign- ments and calculated line positions are indicated. Ketene and t-butylnitrite exhibit continuous or severely broadened one-photon absorption [3] over the wavelengths scanned; at the laser powers used,

2+1 MPI SPECTRUM OF THE

D21-Ii(v=2) - Xzl-I,cv=O,

BAND OF CH

A-doubling not shown

Fig. 1.2 + 1 MPI spectrum of CH from keteoe (upper trace) and t-butylnitrite (lotier trace)_ Marked line positions are cal- culated from constants in table 2. A doubling is not included in the calculation. Lines marked with (a) belong to another band systcm;.the carbon 3p ‘SO + 2p ID2 transition is ma&- cd with @I_

Table 1 Line positions for D 2r$ (u = 2) + X %I~(” = 0) in cm -I a)

N Qll Pll 422 P22

r 64147b) - 63949b)

EIi {

641sb) 64135.

64130

63957b) -

8 c",fE9 63901b) { 64110 64104

63906b)

g GE 63852b) { 64083 64079 631356W

10 { 64049 64054 64043

638oob) 1 64049

63902b)

11 i 64017

63743b) 1 64020

64011 64017 63747b)

12. - 63685") - 63689b)

a) R branch head at 64332 cm-‘. b) AdoubLing unresolved.

that absorption step should saturate, removing any wavelength dependence for the formation of frag- ments that are probed by the same laser pulse. fine petitions are listed in table 1, and were checked against ground-state combination differences [4] _ Upper state constants were extracted by a least-

squares tit and appear in table 2. Of greatest sign% cance is the spin-orbit coupling constant A. Because it is opposite in sign, but approximately the same in magnitude as that for X 21T,, the E state can be given the configuration 2p7r3 _ Assigning E 21Tj to u = 2 of D 2TIi gives a reasonable value for the vibrational fre- quency, and explains the absence of this band in Henberg’s one-photon absorption spectra for CD. Lambdadoubling parameters were not extracted; the

narrowest lines showed a width of 2.5 cm-l - This may be a natural hnewidth, or it may reflect power broadening. In either case, lambda doubling in the R and P branches was not resolved; in the Q branch, it is identifiable, but overlap of broad lines reduces the

Table 2 Constants forD%I,-(u=2)in CITI-'

Th~work Ref. [2]

"0 64209.4 64211.7 Bu 12.7 12.6

D" 15.8X 104 ? A -26.6 ?

406

Volume 121, number 4,5 CHEMICALPHYSJCS LETTERS 15 November. 1985

accuracy of line positions. Values used in the least- squares fit were averages of the two lambda compo- nents.

Several peculiarities remain. Locations for the (1 ,O) and the (3,l) bands in ti-te D + X system can be calcu- lated, estimating the D state vibrational frequency as half the separation between the u = 0 and u = 2 band origins, but neither were, in fact, observed when the laser frequency was scanned through those regions. In contrast to the case with one photon, the D + X tran- sition in two photons is not configurationally forbid- den, so the reason for this observation is presently unclear. Also, the rotational temperature can be esti- mated to be in excess of 3000 K.-A mechanism that produces CH with such excitation is currently under study.

Acknowledgement

Or12 of us (PC) would like to thank the National Science Foundation Graduate Fellowship Program and PfEer Inc. for financial support. We also acknowledge funding from the International Business Ma&&a Cor- poration and the Exxon Educational Foundation.

References

[l] A. Giusti-Suzor and H. LefebvreBrion, Astrophys. J. 2i4 (1977) LlOl.

[2] G. Herzberg and J.W.C. Johns. ANophys. J. 158 (1969) 399_

[3] P. Tarte. J. Chem. Phys. 20 (1952) 1570; WC. Price, JP. Teegao and A-D. Walsh, J. Chem. Sot. (1951) 920.

[4] AE. Douglas and G-A. EBiot, Can. J. Phys. 43 (1965) 496;

I. Botterud. A. Loftus and L. Veseth, Physica Scripta 8 (1973) 218.

407