Infrared spectra of COH2 and COD2 van der Waals complexes in the 4.7 μm region

Volume 186, number I

CHEMICAL PHYSICS LETTERS

1 November 1991

Infrared spectra of CO-H2 and CO-D2 van der Waais complexes in the 4.7 ym region A.R.W. McKellar HerzberglnstitutLoJAstrophysics.

NationalResearch Councilof Canada, Ottawa, Ontario, Canada HA OR6

Received 9 August I99 1; in final form 20 August 1991

Well resolved spectra of CO-hydrogen complexes have been observed in the region of the CO fundamental band using a lowtemperature ( ~50 K) long-path (84 m) absorption cell and a Fourier transform spectrometer. The CO-H2 and CO-D2 transitions appeared aa weak tine structure between the strong lines of the CO monomer. Spectra obtained with the J=O hydrogen5 (para-H, or ortho-D,) resembled perpendicular bands of a T-shapedtriatomic, and on this basis many lines could be assigned. Further assignments and analysis require feedback from detailed calculations using a realistic potential energy surface.

1. Introduction In recent years, there has been tremendous progress in the study of weakly bound molecules by infrared spectroscopy. However, only limited results [ 1,2] have been reported for the complex made up of the two most abundant interstellar molecules, H2 and CO. Although it seems unlikely that conditions in space enable the formation

of significant

quan-

tities of CO-H*, it is still worthwhile to consider this possibility, as done by van den Bout et al. [ 31 in a study of CO dimers. Moreover, spectra of CO-H, are of intrinsic interest in chemical physics and their analysis will lead to a better understanding of the potential surface, which is important for calculations of collisional processes that occur in space. Previous [ 1,2] results for CO-H2 involved spectra accompanying Hz transitions. These were similar to spectra of N,-Hz, and also analogous to spectra of Hz-rare gas complexes [4] with the important difference that the latter can be fully resolved. These CO-H, spectra are very weak since they depend on a small induced dipole transition moment. Thus relatively high pressures must be used, and the resolution is limited by pressure broadening. The high spectral density for CO-H2 (compared, say, to HzAr) made it experimentally impossible to resolve these spectra in the H2 regions. However, other transitions of CO-Hz, with very 58

0009-26 14/91/s

different characteristics, accompany CO transitions. These will be much stronger than those accompanying Hz bands since they depend on allowed transition moments; thus they may be studied at lower pressure and higher resolution. The disadvantage in this case is that the desired CO-H, transitions may be obscured by strong CO monomer absorption. The present paper reports observations of resolved spectra of CO-H2 and CO-D2 complexes in the region of the CO fundamental band. The results were obtained using an 84 m path through CO t hydrogen mixtures at low temperatures, 45-60 K. Distinctly different spectra were obtained using pure para-Hz ci,=O) and normal Hz (25% j,=O, 75% j,= 1). (Here, j, and jco denote rotational angular momenta for the H, (or D,) and CO entities. ) For paraH2 (and ortho-Dz, the j,=O species), only perpendicular type (AK= f 1) spectra were observed, suggesting that the CO internuclear axis is predominantly located perpendicular to the intermolecular axis in the complex. However, the spectra were by no means those expected for a normal semirigid molecule, and it is clear that there is substantial hindered internal rotation of the CO. A potential energy surface for Hz-CO developed by Schinke et al, [ 51, was used recently by Furlong and Danby [ 61 to calculate energy levels and spectra for Hz-CO complexes with j,=O. A complete assignment of the current spectra requires iterative fits 03.50 0 I99

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CHEMICAL PHYSICS LETTERS

using just this sort of calculation. In the present paper, a start is made on the assignment and analysis of the spectra of HI-CO and D2-CO. This should permit a first refinement of the potential surface, leading to further assignments and eventually to the determination of a greatly improved potential. Other complexes containing H2 whose spectra have been studied include (H2)2 [7], Hz-rare gas [8], Hz-N2 [ 21, and H,-HF [ 91. Hz-HF is similar to CO-HI in that transitions are obseped close to allowed lines in the HF fundamental band and very different spectra are observed for j,=O and jH= 1. However, CO-H2 and H2-HF also differ, since anisotropic forces are much greater in the latter. Furthermore, the ratio B(complex)/B(HF or CO) is about 0.04 for Hz-HF and about 0.25 for CO-Hz, leading to very different energy level patterns.

2. Experimental details The experiments were performed with the apparatus used recently for (Hz)* [ 71. The 3.5 m long multiple traversal absorption cell was cooled using a Philips PGH-105 cryogenerator, with temperatures around 50 K obtained by heating the He gas between the cryogenerator and cell using electrical resistance heaters and a Lakeshore DRC-9 1C controller. Spectra were recorded with a Bomem DA3.002 Fourier transform spectrometer fitted with a CaFz beamsplitter and a liquid nitrogen cooled InSb detector. A spectral resolution of 0.013 cm-’ (apodized) was used for the results shown here, and each spectrum was the result of several (3 to 8) hours of data accumulation. Typical pressures for CO and Hz/D2 were 0.1-0.5 Torr and 4-9 Torr, respectively. Temperatures were close to the minimum value consistent with the partial pressure of CO. A large ratio ( > 10) of H2 to CO pressure facilitated low sample temperatures and minimized formation of CO dimers. Tests using pure CO samples confirmed that no detectable traces of CO dimer absorption were present in the CO+ hydrogen spectra. Para-Hz and ortho-D2 were prepared by liquefying the normal gas for a short period in the presence of a chrome-alumina catalyst.

3. Results Under the present conditions, the spectrum in the region of interest is dominated by the strong lines of the “C60 monomer. Fig. I shows the spectrum of a mixture of CO and para-Hz in the central region of thebandcoveringth‘eP(3),P(2),P(l),R(O),R(l) and R( 2) lines of ‘2C’60, each of which completely absorbs over a region of about 0.5 cm - ’ at its center. The next strongest features in the spectrum are sharp lines of the monomers 13C160,W*O, and 12C’70, present in natural abundances of about 1.lo& 0.2%, and O.O4Oh, respectively. The remaining lines in The spectrum, which are numerous but weak (about 5% absorption), are due to CO-H2 complexes. In order to display these more accurateIy, the. spectra were flattened by taking the ratio of the observed spectrum to an artificial background that followed the i2C160 monomer absorption. A resulting flattened spectrum is shown at the top of fig. 1. The gaps in the flattened trace coincide with the regions of maximum monomer absorption. Using normal H2 in place of para-Hz, the strength of CO-H2 lines with j,=O was reduced by a factor

2130

2 140 WAVENUMBER

2150 /cm-’

Fig. I. Observed spectrum of a mixture of para-Hz ( z 8.4 Torr) and CO ( 5 0.5 Torr) at a temperature of 58 K near the CO fundamental band origin. The lower trace is the raw spectrum and the upper trace is an empirical flattened version. The broad lines of ‘%I?0 are labelled on the lower trace. The stronger of the sharp lines are due to other isotopes of CO, as indicated at the top. The remaining weaker lines are due to the CO-H, complex.

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of 4, and a distinct spectrum appeared due to COI-l2with jH = 1 (not shown here). The spectrum obtained with an ortho-Dz t CO mixture is shown in fig. 2, which is plotted in two sections running in opposite directions from an effective band center to reveal the approximate symmetry of the CO-D2 band. Some measured wavenumbers and approximate relative intensities are listed in table 1 (CO-Hz) and table 2 (CO-D,). These give only a selection of the stronger or more significant lines, but even these limited data should provide enough information to determine an improved CO-hydrogen potential. Uncertainties in the data are about f0.002 cm-‘, except for weak lines (Int < 3), where they are somewhat greater. Table 1 includes prominent lines observed between the ‘2C’60 monomer lines P(2) and R(l), about 2135 to 2152 cm-‘. The central region (2140.5-2145.5 cm-‘) is only sparsely populated with weak CO-H2 lines, but these are probably significant and are included in spite of their weakness. Table 1 is presented in two matched columns, taking advantage of the symmetry of the CO-H2 spectrum about a midpoint close to the ‘VO band origin. The appropriate pairing of lines was mostly obvious, but any errors will only be evident when a detailed

2142

2140 WAVENUMBER

2138

2136

/cm-’

Fig. 2. Flattened spectrum of CO-D, obtained in a CO+orthoDz mixture at 49 K with pressures of about 0.3 Torr CO and 5.9 Torr D*. Vertical arrows indicate absorption lines of isotopic CO monomers and the remaining lines are due to CO-D* complexes. The spectrum is presented in two sections, folded about a band center of 2143.07 cm-’ to illustrate the approximate symmetry of the CO-D2 band.

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Table 1 Prominent CO-H2 lines observed in a CO t para-Hz mixture High side wavenumber (cm-‘) 2143.444 2143.495 2143.541 2144.077 2144,331 2144.556 2145.320 2145.536 2146.061 2146.107 2146.191 2146.274 2146.318 2146.519 2146.648 2146.689 2146.859

Low side lnt

1

1 1 1 1 1 1 1 1 5 9 4 10 9 10 4 9

2147.222

15

2147.329 2147.371 2147.632 2147.902 2148.408 2149.339 2149.424 2149.525 2149.598 2149.678 2149.727 2149.789 2149.867 2149.892 2150.135 2150.176 2150.247 2150.300 2150.384 2151.412 2151.558 2151.855

8 5 4 4 3 4 4 3 4 2 1 4 3 4 1 3 4 5 6 4 4 3

wavenumber (cm-‘)

Int

2142.648 2 142.627 2142.610 2142.030 2141.837

1 1 1 1 1

2140.874 2140.610 2140.096 2140.062 2139.981 2139.910 a) 2139.856 2139.643

1 1 1 5 7 2 8 7

2139.166 2139.137 2139.095 2139.075 2138.987 2138.954

6 5 4 3 14 7

2138.829

15

2138.532 2138.251 2137.761 2136.749 2136.649 2136.487 2136.394 2136.290 2136.213 2135.952

4 4 3 2 2 2 2 2 2 4

2135.066 2134.929 2134.680

4 5 2

nI This line is partly obscured by R( 13) of W80 cm-‘.

at 2139.913

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Table 2 Prominent

CO-D2 lines

observedin a CO+ ortho-D2mixture Lowside

Highside wavenumber

Int

2143.301

wavenumber

Int

(cm-‘)

(cm-‘) I 1

2142.828

1 1 1

I

2 142.344 2141.819

2144.659 2145.357 2 146.036

1 1 3

2141.473, 2140.779 2140.090

1 1 3

2146.074 2146.132 2146.207

5 I

2140.053 2 139.997

8 9 I

2 139.925 2 139.837 2 139.803 2 139.736 2139.713

4 6 7

2143.783 2144.314

2 146.298 2146.343 2146.404 2146.463 2146.479 2146.517 2146.629 2146.722 2146.733 2146.817

10 4 3 9 9 7 1

2139.168 2139.118 2139.080 2138.967 2138.929

2 4 4

2138.843 2138.815 2138.648

2149.119 2149.326

3 2

2138.397 2138.219

2149.358 2 149.375 2149.488

4 4

2138.007 2137.638 2137.033

2149.503 2149.748

(4) (4)

______ 6 3 3 4 4

2148.120 2148.728 2148.91 I 2149.024 2 149.09 I

1 8

(3)

______ 2 147.499 2147.678 2147.906

2139.681

8

4 6

2149.801 2149.874 2149.934

7 6 4

2149.970 2150.010 2150.096 2150.159 2150.200 2150.269 2150.324

6 4 5 3 5 5 5

2136.964 2136.937 2136.566 2136.519 2136.314 b, 2 136.288 b’ 2136.240 2136.172 2136.145 2136.085 b, 2 136.027 b, 2135.977

5 a’ 9 5 5 13 13 6 5 4 3 3 3 3 2 2 2 2 5 5 2 3 2 (:) (3)

‘) Below the dashed line, the high-side and low-side columns are not matched (see text )_ b, Indicates an obviously blended line.

1November 1991

Analysis is made. Table 2 also consists of matched pairs of lines for the central region, 2139 to 2147 cm- ‘. Beyond this region, many line pairs could also be observed, but the correct pairing was not so clear, and thus is not given. Weak lines between 2140.5 and 2 145.5 cm - ’ are included because of their likely importacce in the analysis, just as for CO-Hz.

4. Preliminary analysis and discussion The present spectra are approximately symmetric about a midpoint, which is only about 0.2 cm-’ below the ‘2C’60 band origin at 2143.272 cm-‘. This small shift implies that the CO vibrational motion is virtually unhindered in the comfilex. The pervasiveness of the symmetry is a measure of the similarity of the CO-hydrogen intermolecular potential surface for the uco = 0 and 1 states. We know from the spectra and from theory that the internal rotation of H2 or D2 within the complex is quite free. Thus jH is a good quantum number, and complexes with j, = 0 should behave approximately as triatomic molecules. It is instructive to compare these spectra with those of some truly triatomic complexes. The spectrum of CO-Ar has been observed previously using tunable lasers and supersonic jets [ 10,l 1 1, though an analysis is not yet published. COAr may also be observed [ 121 with the present apparatus under conditions similar to those used for CO-Hz. To a good approximation, the CO-Ar spectrum resembles the perpendicular band of a T-shaped triatomic molecule, and subbands may be identified withK,=2t3,1t2,0+1,1t0,2+1,and3+2.Part of this CO-Ar spectrum is shown at the top of fig. 3, which covers the region of the K,= 1+O Q branch (cf. fig. 6 of ref. [ lo] ). The second trace in fig. 3 shows an unpublished spectrum of CO-Ne with a similar K,= 1CO subband. Finally, the two lowest traces of fig. 3 show CO-H2 and CO-D2 spectra in the same region; it is evident that they show analogous Q branches. The CO-H, and CO-D, subband origins are close to one another, and lie between those of CO-Ar and CO-Ne. The evidence of fig. 3 suggests an appropriate assignment of ‘Qo(J), with J= 1, 2, 3, .... for the Q branches observed near 2146 cm-’ in CO-H? and CO-D, (wifhjH=O), and similarly an assignment of 61

Volume 186, number I

CHEMICAL PHYSICS LETTERS

Ar

OR

i, 1450

(, 2146.0

2145.5 WAVENUMBER

21465

/ cm-’

Fig. 3. A comparison of flattened spectra of CO-Ar, CO-Ne, COD2 Cjn=O), and CO-Hz (&=O) in the region of 2146 cm-‘, just below the R(0) line of the CO monomer. The strong line at 2145.05 cm-’ in each traceis R( 7) of “C”O. Experimentalconditions for CO-Ha and CO-Da arc as in figs. I and 2, and those for CO-Ar and CO-Ne are: 6 1 R, 0.3 Torr CO, 4.8 Torr Ar, 0.007 cm-’ resolution; 54 K, 0.3 Torr CO, 6.1 Torr Ne, 0.010 cm-’ resolution. Each trace shows a characteristic K.= I +-0 Q branch whose approximate origin is indicated.

“Q, (J) for the mirror-image Q branches observed near 2 140 cm-‘. Identilication of the accompanying P and R branches is however more difftcult, though it is easy for CO-Ar and CO-Ne. This arises partly because CO-HI and CO-D2 have larger rotational and centrifugal distortion constants, since fewer spectral lines means more difficult pattern recognition, For the present, the only further assignments which could be made with confidence were of the ‘P,(J) and PR, (J) branches for CO-D2. These branches are weak, but they fall in the open central region of the spectrum between CO monomer P ( 1) and R(0). The associated ‘R,(J) and pP,(J) branches should be stronger, but are partly obscured by the monomer lines and are more difficult to assign. For CO-Hz, many of the weak lines observed between 2140.5 and 2145.5 cm-’ also probably belong to ‘P,(J) and PR, (J), but these cannot yet be assigned. The assignments which are reasonably certain are given in table 3. Other regions of the COHz and CO-D2 cjH=O) spectra also show analogies with CO-Ar. For example, clumps of lines around 2150 cm-’ are probably due to the J&=2+1 Q branch.

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In the preceding paragraphs, quantum numbers have been used that are appropriate to a T-shaped near-prolate asymmetric-rotor molecule, namely J, K,, and (implicitly) K, and v,, where ZJ~ denotes the bending vibration. In addition, quantum numbers +-(=1&O), r+i(=OtO), andj,(=O+O, or ltl) were assumed, together with n ( =OtO) for the van der Wards stretch. For example, the transition described as K,=ltO Q(l), or ‘Q,(l), at 2146.107 cm-’ in CO-Hz, is more completely denoted as ( y, J, K,, K,)=(O, 1, l,O)c(O, 1, 0, 1). Thisnotation is not especially linked to a T-shaped asymmetric rotor. In linear molecule notation, this K,= 1~0 subband would be the lI& Z bending fundamental band. In place of these semirigid molecule quantum numbers, a free internal (CO) rotor set can be used. It consists of total angular momentum, J, internal CO angular momentum, jco, and end-over-end angular momentum for the complex as a whole, 1. (The latter should not be confused with the vibrational angular momentum for a linear molecule, also denoted as 1.) The ‘Q, ( 1) transition mentioned above is(J,j,,f)=(l,l,l)t(l,O,l)inthisfreerotor basis. Assignments in terms of both the semirigid molecule and the free rotor quantum numbers are included in table 3. In the T-shaped semirigid limit, the relative line intensities in the spectrum would approximate the perpendicular band of a prolate symmetric top. For the K, = 1c 0 and 0~ 1 subbands, this means a strong Q branch, together with P and R branches whose combined intensity equals that of the Q branch. The branch with AJ= AK, has more strength (e.g. R stronger than P for K,= 1to). In the alternate limit of free CO rotation, only transitions with Ajco= rt 1 and Al= 0 are allowed. For the K,= 1e0 and Oe 1 subbands, this means full intensity for the Q branches and for the branches with AJ=AK,, and no intensity for the branches with AJ= -AK, (which were already the weakest in the semirigid limit), The present CO-D2 spectrum indeed shows that the branches with Ai= -AK,, namely ‘P,(J) and PR, (J), are much weaker than the corresponding Q branches. The same is true for CO-H2 (jr, = 0 ) , since someofthe weakcentrallines (2140.1-2146.1 cm-‘) must belong to these branches even though they are not assigned. Thus the observed intensities are consistent with the interpretation that the CO motion

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Table 3 Assigned transitions in the spectra of CO-HItin= 01ad CO+

(3.1 =O)

Assignment ‘)

Measured wavenumber (cm - I)

semirigid limit

free rotor limit

branch

(Jd,

‘Qo (J)

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CHEMICAL PHYSICS LETTERS

CO-H,

CO-D,

2146.107 2146.191 2146.318 2146.519 2146.859?

2146.036 2146.074 2146.132 2146.207 2146.298 2146.404 2146.517 2146.629 2146.722?

0

(0,1, I,O)~(O, I,O,l) (OJ, 1,I)-(O,2,0,2)

(1313lb-(l,O,I) c&1,2)+(2,0,2)

(0,3, 1,2)+(0,3,0,3) (0,4,1,3)+(0,4,0,4~ (0,5,1,4)+(0,5,0,5) (0,6,1,5)+(0,6,0,6) (037, 1,6)+(0,7,0,7) (0,8,1,7)+-(0,8,O,g) (0,9,1,8)+-(0,9,0,9)

(3, 1,3)+(3,0, 3) (4,1,4)+-(4,0,4) (5, 1, 5)+(5,0, 5) (6, 1,6)+(6,0,6) (7,1,7)+(7,0,7) @,I, 8)*(8,O,g) (9, 1,9)+-(9,0,9)

(0, 1, 1, I)-(O,2,0,2) (‘A.%1,2)+(0,X0,3) (0,3,1,3)+~0,4,0,3) (0,4,1,4)+(0,5,0,3) (0,5,1,5)+(0,6,0,3) (036, 1,6)+(0,7,0,3)

(1,1,0)+(2,0,2) (2,1,1)+(3,0,3) (3, 1,2)+(4,0,4) (4,1,3)+(5,0,5) (5, 134)~ (6,0,6) (6,1,5)*(7,0,7)

2144.659 2143.783 2142.828 2141.819 2140.779 2139.803

‘R,(J)

(0, 1, 1, ~)+(O,O,O,O~ (0,2, 42b-vt I,O, 1)

(1,1,0)+(0,0,0) (2,1, l)e(l,O, L)

2146.479? 2146.817?

‘QI (4

(0,1,O,I)+(O,1,1,0) to,% 0,2)- (0,2,k 1) (0,3,0,3)+(0,3,1,2) (0,4,0,4)+(0,4,1,3) (0,5,0,5)+-(0,5,1,4) (0,6,0,6)+-(0,6,1,5)

(l,O, l)*(l, 1, 1) (2,0,2)+(2,1,2) (3,0,3)+(3,1,3) (4,0,4)+(4, 134) (5,0,5)+(5,1, 5) (6,0,6)*(6,1,6)

pP, (J)

f&O, 0, Ok co,19 1, 1) (0, LO, 1b- (0,2,1,2)

(0,0,0)+(1,1,0) (l,O, I)+-(2,1, 1)

2139.681?

‘RI (J)

(0,2,0,~)~(0,~,~,~) (0,3,0,3)+(0,2,1,2)

(2,0,2)+(1, I,01 (3,0,3)+(2,1, 1) (4,0,4)+(3,1,2) (5,0,5)e(4,1,3) (6,0,6)+(5, 1,4) (7,0,7)+(6,1,5)

2141.473 2142.344 2143.301 2144.314 2145.357 2146.343

‘PO(J)

(0,4,0,4)+(0,3,1,3) (0,5,O,5)+-(0,4,1,4) (0,6,0,6)~(0,5,1,5) (0,7,0,7)+-(0,6,1,6)

between the semirigid and free rotor limits. The 30 assigned transitions of CO-D2 in table 3 enable a limited analysis to be performed. A simple asymmetric rotor rotational energy expression, lies

E,,=B,J(J+1)-D,[J(Jt1)-K2]2 +H,[J(J+1)-KZ]3+A&, A&=*{+(&-C,)J(JSl) +&[J(J+

l)-P]2},

(1)

2140.062 2139.981 2139.856 2136.643

2140.090 2140.053 2139.997 2139.925 2139.837 2139.736

was assumed, where the second term, A&, represents asymmetry splitting and only applies to K- 1. The results of fitting the CO-D2 data of table 3 to eq. (1) are shown in table 4. We do not expect this fit to be too meaningful since CO-D2 is not a simple semirigid molecule. The difference of subband origins in table 4,5.902 cm-‘, is the sum of the uco=O and 1 state &=0-l separations, about twice the A value of the complex (ignoring u-axis centrifugal distortion). If the ratio 63

Volume 186, number 1 Table 4 Fitted molecular parameters for CO-D2 &IO)

& QJ B, Dl H, &-C, d,

(in cm-‘) ‘)

u,=o

U,=l

0.3041(3) (0.0001) b’ 0.2801(g) -0.000129(6) -0.21(6)x10-6 0.1539(10) -0.000511(7)

0.3090(6) (0.0001) b’ 0.2743(5) -0.000138( 10) -0.08(10)x10-6 0.1611(6) -0.000461(7)

origin (K.-l-0) =2146.008(2) origin (K,=O+1)=2140.106(2) ‘) Uncertainties in parenthesesare 1u from the fit. b, Value fixed in fit.

of the A values for vco=O and 1 equals the ratio of B values for the CO monomer, then A(vco=O) = 2.964 cm-‘, A(v,--= 1) =2.938 cm-‘, and the overall band origin is 2 143.070 cm- l. The latter represents a shift of -0.202 cm-’ from the ‘2C’60 monomer band origin. If the complex were rigidly Tshaped, the A value would be equal to the rotational constant of CO, B,= 1.9225 cm-‘, but since CO is partly free to rotate, the moment of inertia is lowered and A is increased. The ground state 5 value of 0.304 cm-’ for J&=0 in table 4 implies an average intermolecular separation of about 3.98 ii for CO-D2. The changes in parameters between the vco= 0 and 1 states are small, illustrating that the potential surface does not change much with CO vibration. The inertial defect given by the rotational parameters is about 30 amu A2, a value which is about 100 times larger than normal, due to the large centrifugal distortion effects in this very nonrigid complex. 5. Conclusions Spectra of the van der Waals complexes CO-HI and CO-D2 have been observed in the region of the

fundamental band of CO using a long absorption path at low temperatures. The spectra observed for j, = 0 were similar to those of the triatomic species CO-Ar and CO-Ne, and it was possible to assign a number of individual transitions. A simple analysis of the CO-D2 spectrum was made, but the resulting parameters have limited meaning since the complex cannot be accurately described by semirigid mole64

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CHEMICAL PHYSICS LETTERS

cule energy level expressions. Further analysis of the present results should be based on detailed calculations using an intermolecular potential surface that is progressively refined. In this way, the improved surface can be used to assign more lines, derive a better surface, and so on. This process would also predict the millimeter wave spectrum of Hz-CO, whose direct observation would be of astrophysical interest and would provide more input data for the calculations. After thej,=O spectra are fitted, then the more challenging j,= 1 data can be tackled in order to give information on hydrogen orientation effects. The present experiment is the first step of a process which should lead to a precise potential energy surface for the CO-hydrogen system. Acknowledgement I am grateful to G. Danby for helpful discussions and for communicating the results of ref. [ 61 prior to their publication. References [ 1] A. Kudian, H.L. Welsh and A. Watanabe, J. Chem. 47 (1967) [2]

Phys.

1553.

A.R.W. McKellar, J. Chem. Phys. 93 (1990) 18.

[ 31 P.A. van den Bout, 1.M. Steed, L.S. Bernstein and W. Klemperer, Astrophys. J. 234 (1979) 503. [4] A.R.W. McKellar, Faraday Discussions Chem. Sot. 73 (1982) 89. [5] R. Schinke, H. Meyer, U. Buck and G.H.F. Di&sen, J. Chem. Phys. 80 (1984) 5518. [6] J. Furlong and G. Danby, Newsletter on heavy particle dynamics, No. 12. Collaborative Computational Project No. 6, SERC. Daresbury Laboratory, ed. J. Tennyson (Dept. of Physics and Astronomy, University Coliege,London, 1989) p. 10. [7] A.R.W. McKellar, J. Chem. Phys. 92 (1990) 3261. [S] A.R.W. McKellar, in: Structure and dynamics of weakly bound molecular cqmplexes, ed. A. Weber (Reidel, Dordrecht, 1987) p. 141, and references therein. [9] CM. Lovejoy, D.D. Nelson Jr. and D.J. Nesbitt, J. Chem. Phys. 87 (1987) 5621; 89 (1988) 7180; K.W. Jucksand R.E. Miller, J. Chem. Phys. 87 (1987) 5629. [lo] A. DePiante, E.J. Campbell and S.J. Buelow, Rev. Sci. Instr. 60 (1989) 858. [ 1I ] Y.P. Zeng, SW. Sharpe, C. Wittig and R.A. Beaudet, 45th Symposium on Molecular Spectroscopy, Columbus, Ohio, June 1I-15, 1990, Paper TA13; private communication. [ 121A.R.W. McKellar, in: Spectral line shapes, Vol. 6, ALP. Conference Proceedings, Vol. 2 16, eds. L. Frommhold and J.W. Keto (American Institute of Physics, New York, 1990) p. 369.