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A supersonic molecular jet for a Fourier transform interferometer: The ν3 band of OCCCO
JOURNAL
OF MOLECULAR
A Supersonic
SPECTROSCOPY
149, 542-556 (1991)
Molecular Jet for a Fourier Transform The v3 Band of OCCCO
Interferometer:
ADAM D. WALTERS, ’ MANFRED WINNEWISSER, KLAUS LATTNER, AND BRENDA P. WINNEWISSER Physikalisch-Chemisches Institut der Justus-Liebig-Universittit, Heinrich-Buff-Ring 58. W-6300 Giessen, Germany A supersonic molecular jet apparatus has been constructed for use with a Fourier transform infrared spectrometer (Bruker IFS 120 HR) or with the millimeter-wave and diode laser spectrometers available in the Giessen laboratory. Design criteria were chosen to provide optimum conditions for obtaining spectral simplification for relatively large molecules with large amplitude motions. Use with unstable species is planned. Thus, throughput could not be too high, and temperature need not be extremely low. First results with the jet system attached to the interferometer are presented. Test measurements with CO2 show a rotational temperature of 22 K for a hole nozzle, and are used to contrast the spectra from a hole and a slit nozzle. A trial jet spectrum of OCCCO shows significant cooling of the low-lying Y, bending mode and considerable simplification, revealing some new features not detected in an earlier room-temperature study. o 1991 Academic Press. Inc.
INTRODUCTION
A Fourier transform spectrometer such as the Bruker IFS 120 HR represents a powerful tool in the infrared study of molecules. The resolution (typically 0.003 cm-’ at around 2000 cm-‘) is such that linewidths are generally Doppler limited. A large spectral range (several 100 cm-’ ) can be covered simultaneously. Even with the wealth of lines which can be collected by this method, the assignment of the spectra of many molecules can prove a tedious and sometimes impossible task in regions of high spectral congestion and overlapping bands. This is true for the cumulenes OCCCO (l-4), OCCCS (5, 6), SCCCS (5, 7, 8), and OCCCCCO (9). In OCCCO the quasilinear u7 vibration associated with bending at the central carbon atom gives rise to an irregular series of energy levels of which 18.25 cm-’ represents the first interval. The effect of this on the rest of the vibrational spectrum is that each fundamental or combination band appears as a complex system of hot bands, sum bands, and difference bands involving v7. High resolution infrared spectra of OCCCO have been reported for the band systems associated with each of the fundamental bands and for various combination bands, as reviewed in Refs. ( I, 10). Most of these band systems could only be partially analyzed. The spectrum of OCCCCCO, which we hope ultimately to measure with high resolution techniques, presents even more serious problems since there are two low-lying ’ Present address: Laboratoire de Spectroscopic Hertzienne, Universitt des Sciences et Techniques de Lille, F-59655 Villeneuve d’Ascq, France. 0022-2852/91 $3.00 Copyright 0 I99 I by Academic Press, Inc. AII rights of reproduction in any form reserved.
542
JET SPECTRUM OF THE vj BAND OF OCCCO
543
bending vibrations, and the lowest-lying mode has not yet been located ( 7). To what extent OCCCCCO is quasilinear is not yet determined. The observed spectrum of this species is further complicated by decomposition products. In addition to being of interest due to their dynamics, the cumulenes and related molecules are good candidates for detection in interstellar clouds ( 11); rotational lines attributable to OCCCS have been identified (12, 13), and the species CCC (Z4), CCCCC (15), and CCC0 ( 11, 16) have been definitely identified in interstellar and circumstellar media. The degree of cluttering in a spectrum can be significantly reduced by cooling in order to reduce the effective number of populated states, the challenge being to do so while maintaining sufficient sample in the gas phase. One answer is the use of a supersonic jet in which the internal degrees of freedom of the molecule are cooled by conversion of internal energy to ordered kinetic energy. Cooling to temperatures of a few kelvin has been demonstrated by laser induced fluorescence techniques which were the first spectroscopic methods to be used with such a jet ( 17). Furthermore since the sample is continuously replenished there is no time for breakdown products to form in significant concentration. The first coupling of a supersonic jet to a Nicolet IT-IR spectrometer was reported by Snavely et al. in 1983 (18) with a resolution limited to 0.06 cm-’ . A short time later Dtibal et al. ( 19) published jet spectra recorded with a Bomem interferometer at a maximum quoted resolution of 0.004 cm-’ using a system with external optics. Later the same group built and used successfully a system with a jet inside the Bomem sample chamber ( 20). The use of a supersonic jet for the study of cumulenes posed theoretical and practical questions. It is well known that in a supersonic expansion the vibrational degrees of freedom are not cooled as efficiently as the rotational. As well as a reduction in the extent of the rotational structure a high degree of cooling of the low-lying bending vibrations is required if the density of hot bands in the spectrum is to be significantly reduced. We believed that these low-lying vibrations should be significantly cooled for two reasons. First, there is a well established trend that low-lying vibrations are better cooled than high-lying ones. Second, these degenerate vibrations are coupled to, or one might say, partake of, molecular rotation, especially in the case of quasilinear species such as OCCCO. We thus decided to construct a jet system for use with the Bruker interferometer, keeping in mind applications to molecules of the size of the cumulenes discussed above and to possible unstable species. This paper reports on the design of our system. test spectra obtained with the spectrum of COz, and an experiment with a quasilinear molecule of only moderate stability. OCCCO. For the initial studies of unknown spectra we chose the IQband system of OCCCO, which is the strongest band in the spectrum and falls where the spectrometer is most sensitive. It was also chosen because it lies near the v4 band of OCCCCCO at 22 13 cm-- ’ , as can be seen from Fig. 2a in Ref. ( 9). Despite the difficulties previously discussed, the u3 band system of OCCCO was to a large extent assigned by Fusina ut ul. ( 1) from a room-temperature spectrum recorded with the high resolution laboratory interferometer at the Laboratoire d’Infrarouge at Orsay. They had difficulties assigning lines in the most congested regions, which leaves gaps in the analysis. The simplification achieved with our preliminary experiment using the jet allowed new information to be obtained, complementing the results of the room temperature study.
544
WALTERS ET AL. EXPERIMENTAL
DESIGN DETAILS
We considered carefully a solution analogous to the later Zurich arrangement, placing the jet within the sample chamber of the interferometer system (20). It was decided, however, for several reasons, not to follow such a scheme but instead to construct external optics to transfer the parallel IR beam coming directly from the interferometer optics to the jet. First, the optical axis in the sample chamber of the Bruker instrument lies closer to the ground than that of the Bomem so that there is insufficient space to mount a large diffusion pump. Second, the jet system was to be designed to allow its possible use with millimeter-wave or diode laser spectrometers. Third, the external jet chamber allows for easier introduction of other elements into the experiment such as a multipass cell, or a laser beam for in situ production of unstable molecular species by means of photodecomposition (21) . Figure 1 shows a schematic diagram of the finished system. The overall configuration was determined by the narrow space in the laboratory and along the access path to the interferometer. The system is “portable” on a single cart and can be flanged directly onto the parallel beam output port of the interferometer. Only the roughing pump is mounted separately. A major practical problem is the provision for use with unstable molecules. All connections to the nozzle were made in PFA Teflon and the pressure measured with a piezocrystal gauge. In this way contact of the sample with metal surfaces is minimized. Problems have been reported (22) with the blocking of pulsed nozzles by molecules such as SCCCS owing to cluster formation combined with decomposition. The simple open hole required in any case for the continuously scanning interferometer was expected to be advantageous in this respect. For the initial experiments a metal nozzle was used, but this can later be replaced by a glass nozzle.
spherical
mirror 1’
f=720
i
mm
spherical
mirror
f=280
plane
mirror
(detector plane
_’
-
Bruker
FT-IR
mm
bellows
diffusion
out of
of
drawing)
pump
Spectrometer
FIG. 1. Schematic diagram of molecular jet apparatus and associated optics designed for coupling to the Bruker HR 120 IFS. The nozzle axis is perpendicular to the plane of the figure.
JET SPECTRUM
OF THE v3 BAND OF OCCCO
545
The choice of pump, in particular whether to use a Roots blower or oil diffusion pump, was the subject of considerable initial consideration. For removal of large quantities of gas at above 10e2 mbar a Roots blower is most efficient. An immediate technical problem, however, is to couple the pump to the nozzle chamber without significant loss in pumping speed while avoiding transmission of pump vibrations to the optical system. An undesirable addition to the jet-cooled spectrum is a warm backgroundgas spectrum caused by molecules rethermalized by collisions with the walls of the vacuum chamber. In order to reduce this background spectrum the chamber pressure should be maintained as low as possible. The price and size of Roots blowers with sufficient speed to maintain a pressure below lO-2 mbar with the nozzle open rise steeply as the required pressure is lowered. Contrarily the pumping speed of most diffusion pumps decreases rapidly for pressures exceeding low3 mbar and the pump will soon stall if operated significantly above this limit. The huge physical dimensions and prices of appropriate oil booster pumps make them unsuitable. A 6000-liter set ’ Leybold “Ejector” pump was chosen. This pump spans a pressure range which allows the optimum choice between large sample flow rates and reduced background pressures. It also gives the possibility of switching to a pulsed nozzle system with much lower chamber pressures at a later stage. In the high pressure region this pump requires backing by a pumping system with a speed of at least 200 m3 hr-‘. The combination of an Edwards EHSOOA Roots pump and a E2M80 rotary pump is used to provide the necessary fore-vacuum. For speed of adjustment and in order to protect optical parts during pump warm-up the vacuum chamber can be isolated from the diffusion pump by means of a gate valve. The use of a large diameter (320 mm) valve and special cold-cap rather than traditional baffle results in negligible reduction of the pumping speed. In addition to one port for the nozzle and two ports for the IR beam the vacuum chamber contains five additional vacuum ports through which, for instance, laser radiation can be brought to the jet. Pressure in the chamber is monitored by a Pirani-Penning gauge combination. The parallel IR output beam of the FT-IR spectrometer is focused at the jet by means of a spherical mirror (f = 720 mm) placed at a low angle of incidence ( 11’ ) to limit astigmatism. For a source aperture of 1 mm a focused beam diameter of 2 mm at the jet is obtained with f/10 optics. The diverging beam is then focused on the detector by another spherical mirror (f= 280 mm). The optical path is under vacuum, open to the interferometer vacuum, and pumped separately from the expansion chamber. The IR radiation traverses the chamber through two windows which are only as far from the jet as is necessary to prevent jet disruption hence reducing the problem of a warm background spectrum of scattered molecules. By the use of bellows the window positions can be varied. The nozzle is also adjustable in position along the jet axis, transverse to the IR beam. Allowance has been made to displace the jet 6 cm from the plane of the IR beam in order to accommodate the planned future addition of a multipass optical unit. Two nozzle configurations have been tried so far: a hole nozzle with a diameter of 0.3 mm, and a slit nozzle 10 by 0.01 mm. The measurements reported here were made with a globar source, an InSb detector, an optical filter for the region 1900-2300 cm-’ , and resolution [ 1 / ( maximum optical path difference)] as high as 0.0018 cm-’ . Lower resolution was used for the OCCCO
546
WALTERS ET AL.
spectrum, as is discussed below. The sample of OCCCO was prepared and redestilled from the dehydration of malonic acid with P205. PERFORMANCE
TESTS WITH COz
The system was initialized with measurements of the antisymmetric stretching fundamental band of CO* at 2350 cm-‘. Test measurements were made at various pressures both with undiluted substance and with a seeded argon jet, with different positions of the nozzle relative to the axis of the optical beam, and for both hole and slit nozzles. Figure 2 shows the infrared (naperian) absorbance spectrum of the band with COZ expanded from 500 mbar, undiluted, through the 0.3-mm hole nozzle. Fifty scans
3
al CO,
in nozzle
jet
I
2320
b)
CO2
at room temperature
cl
Jet
spectrum
2330
minus room temperature spectrum
2340
2350
2360
2370
i
2 3f 30
Wavenumber/cm-'
FIG. 2. Spectrum of CO2 expanded from 500 mbar through the 0.3-mm hole nozzle. Fifty scans were taken, each with a S/N ratio of 25 / 1. The three spectra show (a) the raw data in naperian absorbance, (b) the absorption due to room-temperature gas at the pressure (0.001 mbar) of the background in the chamber, and (c) the jet spectrum obtained by subtracting (b) from (a).
JET SPECTRUM
547
OF THE yj BAND OF OCCCO
were taken although the spectrum was easily observable after one scan with a maximum signal-to-noise ratio (S/N) of 25 / 1. The line profile of the R( 0) transition is shown in Fig. 3a to have a width (FWHM) of 0.008 cm-’ dominated by the divergence of the jet parallel to the path of the IR beam. The central dip in the line profile, consistent with previous observations, suggests inhomogeneous cooling across the jet (23). Slight side lobes of the instrumental sine function can be seen to either side of this line, indicating that the resolution employed, 0.004 cm-‘, was insufficient to display the steep flank of the actual lineshape. The line profile of the R( 20) transition shown in Fig. 3b, in contrast, looks like a Doppler-broadened line and has a width of only 0.006 cm-‘, showing no side lobes. The higher J transitions would appear therefore not to originate from the expanding gas. A spectrum was hence taken while effusively leaking COz into the chamber with the diffusion pump running. The chamber pressure was
1
0
Wavenumber/cm-'
d)
R (8)
Cl
1
2.0-
R (81
2.0-
: 6 0 R
+
l.O-
CO.003
cm-'
+
l.O-
0.006
cm-'
4
o.o-
I
o.o-
/
2355.88
I 2355.90
Wavenumber/cm-'
I
L 2355.88
2355.90
Wavenumber/cm-'
FIG. 3. Line profiles of CO2 obtained with the molecular jet: (a) hole nozzle 0.3 mm in diameter, stagnation pressure 500 mbar, resolution 0.004 cm-‘, (b) warm gas in nozzle spectrum, resulting from background pressure of 0.001 mbar, (c) slit nozzle 10 X 0.01 mm, stagnation pressure 90 mbar, resolution 0.0023 cm-‘. (d) slit transverse to infrared beam, stagnation pressure 90 mbar, resolution 0.004 cm-’
548
WALTERS
ET AL.
maintained at the same pressure ( 1 X 1Oe3 mbar) observed when the jet was running. The resulting spectrum of 50 accumulated scans is shown on the same scale in Fig. 2b. The measured linewidth is also 0.006 cm-’ and the high J lines are the same intensity as the high J lines in the jet spectrum so that the origin of the latter is explained. (The Doppler width of these lines at room temperature should be 0.0044 cm-’ ; the observed linewidth corresponds to the convolution of the instrumental lineshape with the Doppler lineshape.) Figure 2c shows the CO;! jet spectrum with the background-gas spectrum directly subtracted; no further manipulation or scaling has been used. Comparison with simulated rotational distributions indicate that the rotational temperature of this spectrum is 2 1 K, although the peak intensity distribution is slightly non-Boltzmann, with the ratio of R( 0) to R( 2) giving a temperature of 19 K and that between R(0) and the higher J lines giving 23 K. The inhomogeneous cooling, mentioned above in connection with the lineshape, would easily account for this range of apparent temperatures. The results shown in Fig. 2 and scans (a) and (b) of Fig. 3 were taken with the center of the focused IR beam 2 mm downstream of the nozzle. With the nozzle closer, strongly non-Boltzmann rotational distribution is seen. With the nozzle further displaced, the rotational temperature in the jet is slightly lower but the warm background observed significantly increases. Spectra were also recorded using a slit nozzle approximately 0.0 1 mm in width and 10.0 mm long, lined up parallel to the IR beam. The area of its opening is roughly equivalent to that of the hole nozzle discussed above. The linewidth was reduced to 0.003 cm-’ as shown in Fig. 3c. The reduction in linewidth shows that the spreading of the jet along the optical axis is significantly reduced. The rotational distribution obtained with a stagnation pressure of 90 mbar (at higher pressures, the signal was so strong that the strongest lines had zero transmittance) could not be matched exactly to any one temperature but shows the greatest similarity to a calculated spectrum for 75 K. When the slit is placed transverse to the IR beam the linewidth, shown in Fig. 3d, increases to 0.006 cm-‘. Interestingly this is still narrower than the linewidth observed with the hole nozzle and the line profile does not show any central dip. The rotational temperature is reduced to 50 K leading to the conclusion that the slit is cooler in the middle than at the ends. Inhomogeneous cooling along the slit is of course consistent with a distribution which is not fully Boltzmann in the spectrum with the slit parallel to the optical axis. The CO2 rotational temperature observed with the slit nozzle parallel to the optical axis is higher than that with the hole nozzle, but for equivalent sample throughput we gain both intensity and resolution, as expected. The optimal nozzle configuration will depend on the sample and the transition intensity to be studied. RESULTS
WITH
OCCCO
The u3 band of OCCCO selected for our first experiment was measured at room temperature, at a resolution of 0.003 cm-’ by Fusina et al. ( I). (The Doppler width is 0.0033 cm-’ .) They were able to assign a large proportion of the observed rovibrational lines between 2000 and 2500 cm-‘. In the less dense sum and difference band regions of this range, the assignments included lines from the sixth excited state of
JET
SPECTRUM
OF
THE
yj BAND
549
OF OCCCO
the u7mode. However, in the dense region of the strong transitions with Av7 = 0, they were not able to identify transitions coming from levels with v7 higher than 2. We recorded this band with the jet system using the hole nozzle. The sample was undiluted, at a stagnation pressure of approximately 250 mbar. The quantity of available sample limited the number of recorded scans to seven, or approximately 15 min total measuring time, so that the spectrum presented here is not typical of the S/N that can be obtained with reasonable measuring times. Because of the double-peaked lineshape discussed above, we recorded the OCCCO spectrum at a resolution of 0.008 cm-‘, which led to single-peaked lines broader than the Doppler width. Since the observed spectrum was not dense, the relatively low resolution of the spectrum was not a disadvantage. Figure 4 shows the overview of a room-temperature spectrum and the jet spectrum, transformed at low resolution. The drastic change in the intensity distribution is clearly seen. Hot bands coming from levels up to v7 = 4, but no higher. can be seen in the jet spectrum. Indeed, even in the low resolution spectrum the Q branches of the hot bands for l,> 1 can be distinguished; these Q branches could not be identified at all in the room-temperature spectrum ( I ) . The observed and assigned bands of the v3 band system are indicated in Fig. 5, which shows the energy levels of the v7 stack in the ground state and with one quantum of u3 excited. The rotational distribution is shown in Fig. 6 for the observed spectrum and a simulation of the fundamental band. This distribution defines a rotational temperature Trot of 60 K. with an uncertainty of +2 K. The simplification of the spectrum is shown in detail in Fig. 7, in which the overlap region between the fundamental and the first hot band is shown for the jet spectrum and a room-temperature spectrum. The new information that was determined from this spectrum, that is, rovibrational
I
-
I
a) :E 1 s
Cell
at
300
K
00
z
2
0.80
+F 0.60
I 2250 Wavenumber/cr.'
FIG 4. Low resolution overview of q fundamental band of OCCCO recorded (a) at room temperature (widely spaced lines at lower wavenumber are CO* impurity) and (b) with the molecular jet system.
550
WALTERS ET AL. 2430 2410
2370
31 33 20 22
FIG. 5. Energy level diagram of the u, vibrational stack of OCCCO in the ground state and with one quantum of the yj mode excited. The bands observed and assigned in the jet spectrum are indicated with vertical lines.
which could not be identified in the analysis of the room-temperature spectrum due to crowding, include the hot bands v3 + 3~: - 313 and v3 + 4v$ - 4v+, the Q branches for the whole series v3 + nv; - nv; for n = 1 to 4, and the low J lines of the first hot band, v3 + Y: - vj. This first hot band is shown in the Loomis-Wood diagram (24) of Fig. 8, and is seen to be strongly shifted by a local resonance affecting both components. The Q branch is displaced by 0.01 cm-’ from the position derived from fitting only the high J lines. The observation of this interaction may help in the identification and interpretation of the set of local resonances observed already for several of the other states in the v3 + 12v7 manifold in the work of Fusina et al. (I ). The line positions for the three hot bands discussed above are given in Tables I and II. The wavenumbers C(J”) for the v3 + 3~: - 3~: and v3 + 4vi: - 4~; bands were fitted to the expression
lines
;( J”) = Ec( J’) - Ec( J”)
(1)
JET SPECTRUM
551
OF THE vj BAND OF OCCCO
0.4 b)
Simulated.
60
K
FIG. 6. Fundamental ~3 band of OCCCO (a) recorded with a molecular jet and (b) simulated for a rotational temperature of 60 K.
with
E,(J) = Gc(v, 1) + B,[J(J + 1) - 12] - D,[J(J
-t 1) - 12]2 + If,[J(J
+ 1) - 1213
(2)
to obtain effective constants for these two subbands. The band origin is given by the difference between the vibrational term values G,( u, I), SO= GC(u’, 1’) - G,.( v”, I”).
(3)
4.0 b) ,”
2
I
1
3.0
D L
2.0
E Q
1.0 no
I 2277
I
I
2278
I
2279
I 2280
Wavenumber/cm-'
FIG. 7. Excerpt of the spectrum of OCCCO in the overlap region of the ~3fundamental and its first hot band from the u, vibrational level, strongly displaced to lower wavenumber. recorded with (a) a molecular jet and (b) at room temperature.
552
WALTERS
ET AL.
l
4
n :r-34,441
Freq.:C
2277.009,
2289.3631
Peaklist
E ZTGSCSl
FIG. 8. Loomis-Wood diagram showing the identification of the low J lines of the e and fcomponents of the first hot band, v, + Y{- Y:, in the spectrum of OCCCO. The weak Q branch is indicated.
This is the same definition of effective constants as that used by Fusina et al. ( 1)) Karyakin et al. (25), and Vander Auwera et al. (4) for the center of each set of ldoublets. For each subband, the lower state constants were held fixed at the effective constants obtained from the results of Vander Auwera et al. (4). Due to the perturbation in the v3 + v$ - v$ hot bands, such a fit is not presented for these subbands; we could not improve on the attempt by Fusina et al. (I) to determine unperturbed constants. The positions of the four Q branches observed and the effective constants for the upper states of the two new subbands are listed in Table III. From the above definitions we find that the Q-branch transition with the lowest possible J value, J = 1, which dominates these weak parallel Q branches, is given by Co = io + (B’ - B”)I.
(4) The calculated and observed Q-branch positions agree within the estimated error of the positions of the Q branches, which are only slightly wider than single lines. The line positions and effective constants obtained for the v3 fundamental and the hot bands for v: = 2’ and vi = 22 agree very well with the data obtained by Fusina et al. (I), and are not repeated here. A rough measure of the vibrational cooling of the v7 excitation could be obtained from the relative intensities of the hot bands and the fundamental band ~3. Assuming that the transition moment is the same for the excitation of one quantum of v3, independent of the excitation of v7, the intensity of the lines for a fixed J in the successive hot bands should reflect the vibrational temperature. Lines for J = 12 and 20 were selected and compared, the HBnl-London factor which depends on I was considered, and the vibrational temperatures listed in Table IV were obtained. The large estimated error for v = 1 = 4 reflects decreasing signal-to-noise ratio of the lines used. The resulting vibrational temperatures appear to decrease with increasing excitation of v7. Since the opposite trend would seem more probable, in view of what is
JET SPECTRUM
OF THE v3 BAND
553
OF OCCCO
TABLE I
Observed Line Positions of the u, = I = 1 Hot Band of vj of OCCCO v = l,l=
v=l,l=l’
J”
I’( J”) /cm-’
1 3
2281.16767
R( J”) /cm-l
J”
P( J”) /cm-’
lf
R( J”) /cm-l
2281.92838
2
2281.31704
2282.088 18
2282.240 49
4
2281.01645
2282.405 03
5
2280.870 50
2282.559 88
6
2280.72178
2282.727 56
7
2280.58102
2282.860 54
8
2280.432 47
2283.058 18
9
2280.272 93
2283.184 27
10
2280.150 56
2283.369 81
11
2279.988 22
2283.509 57
12
2279.850 54
2283.707 23
13
2279.704 75
2283.837 54
14
2279.576 23
2284.047 22
15
2279.42382
2284.168 51
16
2279.304 31
2284.390 72
17
2279.14630
2284.502 82
18
2279.036 06
2284.737 86
19
2278.872 25
2284.840 11
20
2278.77171
2285.089 26
21
2278.60128
2285.180 54
22
2278.51176
2285.444 77
23
2278.333 33
2285.525 17
24
2278.255 89
2285.804 14
25
2278.06908
2285.870 85
26
2278.004 24
2286.167 17
27
2277.807 72
2286.219 70
28
2277.756 81
2286.534 11
29
2277.549 78
2286.573 37
30
2277.513 26
2286.905 31
31
2277.294 61
2286.929 02
32
2277.273 79
2287.28189
33
2277.040 24
2287.28189
34
2277.040 24*
2287.658 68
35
2276.794 02
2287.648 77
36
2276.806 02
2288.040 59
37
2276.547 74
2288.012 93
38
2276.57834
2288.426 23
39
2276.303 18
2268.378 82
40
(2276.353 43)
2288.814 21
41
2276.064 24
2288.748 39
42
2276.133 90
(2289.203 33)
43
2275.826 54
2289.120 39
44
2275.91648
2289.598 24
45
2275.592 87
(2289.492 69)
46
2275.700 34
2289.997 15
47
2275.359 75
2289.87096
48
2275.489 21
2290.395 73
49
2275.130 59
2290.248 09
50
2290.796 17
51
2274.902 52
2290.62791
52
2291.19127
53 55
(2291.01105) 2291.393 36
understood concerning cooling in jets, we conclude that our assumption is not valid, and that the transition moment for one quantum of u3 depends sensitively on the excitation of the quasilinear bending mode u7. From Fig. 5 it can be seen that the pattern of the energy levels in the upper u7 vibrational stack is notably different from the ground state stack. The coupling between v7 and v3 indicated by this difference makes it indeed quite likely that the transition moments of the fundamental v3 band and of the hot bands are different. This may also be the reason that only a few lines of the 2,: = 3 ’ hotband were distinguishable, although they should be stronger than the II: = 44 hot band, as can be seen from Fig. 5. The real vibrational temperature of even the first hot band may therefore be higher than that shown in Table IV: the listed temperatures are probably all lower than the true excitation temperatures.
WALTERS
554
ET AL.
TABLE II
Observed and Calculated Line Positions of the u, = I = 3 and 4 Hot Bands of Y)of OCCCO W =3,1=3 J”
P( J”) /cm-’
3 4
v,=4,1=4 R(Y)
(0 - c) /lo-%m-1
(2267.39033)
/cm-1
(0 - c) /IO-“cm-’
2268.63184
-14
2268.78916
P( 3’) /cm-’
R(J”)
(0 - c) /lo-%n-’
(0 - c)
fcui
/IO-&cm-’
(2262.95741)
21
5
2267.235 10
-181
2268.94582
6
2267.082 82
-107
2269.10469
62
(2261.24082)
(2263.27740)
7
2266.S3095
2269.262 50
29
(2261.08843)
(2263.43829)
a
2266.77961
57
2269.42137
66
2260.93634
-30
2263.59832’
9
2266.62802
81
2269.57976
19
2260.78497
-50
2263.762 59.
10
2266.475 86
11
2269.73877
2
2260.63487
-2
2263.92630*
11
2266.32562
95
(2269.898 22)
2260.48596
103
2264.08637
-132
12
2266.17403
11
2270.05832
2260.33804
249
2264.25006
-141
13
2266.024 14
66
2270.21764
-21
2260.18594
-82
2264.41587
14
2265.87360
29
2270.37737
-51
2260.03732
-124
2264.58016
2259.89040
-55
2259.74385
-5
2264.91165
-31
-50
(2263.11710)
(2261.39382)
38
-145 76 183
5 -57 -1
2264.746 17
15
2265.72337
2
2270.538 15
16
2265.57392
39
(22700.69787)
17
2265.42329
-46
2270.85771
2259.59867
123
2265.07869
-1
18
2265.273 66
-25
2271.01643
-52
2259.45108
-46
2265.24725
150
19
2265.12349
-38
2271.17505
-66
2259.30692
72
2265.41390
57
20
2264.973 17
-30
2271.33378
2259.16132
-9
(2265.58139)
-47
24 10
10
2271.49026
-32
2259.01718
2271.64610
-2
2258.87332
2265.751 W
104
2265.91860
-42
15
2266.08798
-57
72
2266.25864
2
21
2264.82206
22
2264.67083
23
2264.51803
-4
(2271.79995)
2258.73043
24
2264.364 40
40
(2271.95168)
2258.58835
25
2264.20733
-92
2272.10132
45
2258.445 11
-37
26
2264.05165
121
2272.24666
-36
2258.303 76
27
2263.89037
24
2272.38969
2258.16364
28
2263.72666
2258.02275
2
14
2272.52648
-16
-138
-14
-25
-9
2266.59899
-89
95
2266.77127
7
73
2266.94308
15
2257.88392
210
2267.11468
2257.74168
-40
2267.28765
134
2267.459 56
29
2263.56191 2263.38743
31
2263.21486
195
2257.60412
32
2263.03040
-90
2257.46383
-8
2267.63255
2257.32407
-140
(2267.80733)
197
33
10
2266.428 74
30
-143
-53
34
2257.18790
35
(2257.04979)
36
2256.91164
47
-38 7 -92 -118 -172
2267.979 53
38
2268.155 86 -88
5
2268.33007 (2268.50483)
37
109
38
2268.68099
39
(2268.855 20) 53
2269.03127
40
* Lines marked with an asterisk were not used in the fit. TABLE III New Constants Obtained from the u, = I = n Hot Bands of ~3of OCCCO”’ U, = 1,
i&j/cm-’
b,
2281.6229
2
2274.4626
3
2268.0116
2268.009 77 (29)
0.0777016 (31)
16.9 (83)
4
2262.1763
2262.17287 (31)
0.078332 7(10)
49.60 (64)
a) Lower state constants B” = 0.0775043535 B”
=
D’/lO-scm-’
B’jcm-’
&/cm-’
1
0.078027664
b, The listed
were taken from Vader
cm-‘, cm-‘,
D” = 3.22291 D” = 3.19442
values are observed
x lo-’
-0.243 1(59) _
a/10-3cm-’ _ 0.75 0.89
Auwera et al., Ref. (4): for v = 1 = 3, cm-‘,
x 10es cm-‘,
peak positions.
H’/lO-gcm-’
If” = 1.5708 x lo-l3 II” = 1.3862 x lo-l3
cm-‘,
and for v =
cm-‘.
Note that &J = ia + ABI for the lowest J value.
1= 4,
JET SPECTRUM
OF THE v, BAND
OF OCCCO
555
TABLE IV
Vibrational Temperatures Derived from Line Intensities in the Molecular Jet Spectrum of OCCCO Hot band of “3 Ratio of intensity of hot band to 07,/7 ~3 fundamental 0.788 1, 1
Effective81 Tv;b
101 (IO)
2, 2 2, 0
0.430 0.255
79 (5) 64 (5)
3, 3 434
0.160 0.085
63 (5) 70 (20)
%erived
assuming that the transition moments of
hot bands are the same as for the v3 fundamental
CONCLUSIONS
The vibrational temperatures in the jet spectrum of OCCCO shown in Table IV may be considered a lower limit. If we drop the assumption of constant transition moments with excitation of v7, and assume rather that the v7 vibrational stack should show a consistent vibrational excitation temperature, we estimate the vibrational temperature in the v7 bending manifold to be 120 (+20) K. or about 60 K above the rotational temperature, which was 60 K. The degree of cooling obtained with a supersonic jet in the bending manifold of a molecule such as OCCCO, even working with the undiluted sample, is thus amply sufficient to simplify the spectrum drastically while still retaining a large amount of information. The initial results suggest that the system will be very valuable for the study of other cumulenes, or indeed any molecule with a low-lying vibrational mode. Modifications now being implemented include variation in the length of the slit nozzle to optimize sample usage, intensity, and linewidth. It should be possible to obtain significant new information from the bands of OCCCO and the spectra of other cumulenes and other species of a comparable size or complexity. ACKNOWLEDGMENTS
We are grateful to Professor M. Quack and his group for constructive discussion prior to the design of the Giessen system, and Dr. K. M. T. Yamada for discussions concerning design and lineshape. We thank Dr. R. Wilmes for the preparation of a sample of OCCCO. A.D.W. acknowledges a fellowship of the Royal Society. This project was supported by funds from the Deutsche Forschungsgemeinschaft and the Fonds der chemischen Industrie. RECEIVED:
June
10, 1991
REFERENCES 1. L. 2. P.
FUSINA.
1.
M. MILLS, AND G. GUELACHVILI, J. Mol. .Spmro~c 79, 101-122 ( 1980)
JENSEN,J. Mol. Spectrosc. 104, 59-71 ( 1984). 3. P. JENSENAND J. W. C. JOHNS, 1. Mol. Specrrosc. 118.248-266 ( 1986).
556
WALTERS
ET AL.
4. J. VANDER AUWERA, J. W. C. JOHNS,AND 0. L. POLYANSKY,J. Chem. Phys., in press, 1991. 5. F. HOLLAND, M. WINNEWISSER,AND J. W. C. JOHNS,Can. J. Phys. 68,435-448 ( 1989). 6. F. HOLLAND AND M. WINNEWISSER,.I. Mol. Spectrosc. 147,469-5 12 (1991). 7. F. HOLLAND, M. WINNEWISSER, C. JARMAN,H. W. KROTO, AND K. M. T. YAMADA, J. Mol. Spectrosc. 130, 344-370 ( 1988). 8. F. HOLLAND AND M. WINNEWISSER,.I. Mol. Spectrosc. 147, 496-5 12 ( 199 I ). 9. F. HOLLAND, M. WINNEWISSER,G. MAIER, H. P. REISENALJER, AND A. ULRICH, J. Mol. Spectrosc.
130,470-474 (1988). 10. B. P. WINNEWISSER,in “Molecular Spectroscopy: Modem Research” (K. Narahari Rao, Ed.), Vol. III, Chap. 6, Academic Press, New York, 1985.
11. H. E. MATTHEWS,W. M. IRVINE,P. FRIBERG,R. D. BROWN, AND P. D. GODFREY, Nature (London) 310, 125-126 (1984). 12. B. E. TURNER, Astrophys. Lett. 23, 217-224 ( 1983). 13. H. E. MATTHEWS, J. M. MACLEOD, N. W. BROTEN, S. C. MADDEN, AND P. FRIBERG,Astrophys. J. 315,646-653 (1987). 14. K. W. HINKLE,J. J. KEADY, AND P. F. BERNATH,Science 241, 1319-1322 (1988). 15. P. F. BERNATH,K. H. HINKLE,AND J. J. KEADY, Science 244,562-564 ( 1989). 16. R. D. BROWN, P. D. GODFREY, D. M. CRAGG, E. H. N. RICE, W. M. IRVINE,P. FRIBERG,H. SUZUKI, M. OHISHI,N. KAIFLJ,AND M. MORIMOTO,Astrophys. J. 297.302-308 ( 1985). 17. R. E. SMALLEY,L. WHARTON, AND D. H. LEVY, Ace. Chem. Res. 10, 139-145 (1977). 18. D. L. SNAVELY, K. B. WIBERG, AND S. D. COLSON,Chem. Phys. Lett. 96,319-323 (1983). 19. H.-R. DOBAL, M. QUACK, AND I-J.SCHMI~, Chimiu 38,438-439 (1984). 20. A. AMREIN, M. QUACK, AND U. SCHMITT, J. Phys. Chem. 92,5455-5466 ( 1988). 21. H. KANAMORI,J. E. BUTLER,K. KAWAGUCHI,C. YAMADA, AND E. HIROTA, J. Chem. Phys. 83,61 l615 (1985). 22. K. M. T. YAMADA, private communication. 23. M. HEPP, Diplom thesis, Universitlt zu KijIn, 1990. 24. B. P. WINNEWISSER,J. REINSTADTLER,K. M. T. YAMADA, AND J. BEHREND,J. Mol. Spectrosc. 136, 12-16 (1989). 25. E. N. KARYAKIN, A. F. KRUPNOV,AND S. M. SHAPIN,J. Mol. Spectrosc. 94,283-301
(1982).
OF MOLECULAR
A Supersonic
SPECTROSCOPY
149, 542-556 (1991)
Molecular Jet for a Fourier Transform The v3 Band of OCCCO
Interferometer:
ADAM D. WALTERS, ’ MANFRED WINNEWISSER, KLAUS LATTNER, AND BRENDA P. WINNEWISSER Physikalisch-Chemisches Institut der Justus-Liebig-Universittit, Heinrich-Buff-Ring 58. W-6300 Giessen, Germany A supersonic molecular jet apparatus has been constructed for use with a Fourier transform infrared spectrometer (Bruker IFS 120 HR) or with the millimeter-wave and diode laser spectrometers available in the Giessen laboratory. Design criteria were chosen to provide optimum conditions for obtaining spectral simplification for relatively large molecules with large amplitude motions. Use with unstable species is planned. Thus, throughput could not be too high, and temperature need not be extremely low. First results with the jet system attached to the interferometer are presented. Test measurements with CO2 show a rotational temperature of 22 K for a hole nozzle, and are used to contrast the spectra from a hole and a slit nozzle. A trial jet spectrum of OCCCO shows significant cooling of the low-lying Y, bending mode and considerable simplification, revealing some new features not detected in an earlier room-temperature study. o 1991 Academic Press. Inc.
INTRODUCTION
A Fourier transform spectrometer such as the Bruker IFS 120 HR represents a powerful tool in the infrared study of molecules. The resolution (typically 0.003 cm-’ at around 2000 cm-‘) is such that linewidths are generally Doppler limited. A large spectral range (several 100 cm-’ ) can be covered simultaneously. Even with the wealth of lines which can be collected by this method, the assignment of the spectra of many molecules can prove a tedious and sometimes impossible task in regions of high spectral congestion and overlapping bands. This is true for the cumulenes OCCCO (l-4), OCCCS (5, 6), SCCCS (5, 7, 8), and OCCCCCO (9). In OCCCO the quasilinear u7 vibration associated with bending at the central carbon atom gives rise to an irregular series of energy levels of which 18.25 cm-’ represents the first interval. The effect of this on the rest of the vibrational spectrum is that each fundamental or combination band appears as a complex system of hot bands, sum bands, and difference bands involving v7. High resolution infrared spectra of OCCCO have been reported for the band systems associated with each of the fundamental bands and for various combination bands, as reviewed in Refs. ( I, 10). Most of these band systems could only be partially analyzed. The spectrum of OCCCCCO, which we hope ultimately to measure with high resolution techniques, presents even more serious problems since there are two low-lying ’ Present address: Laboratoire de Spectroscopic Hertzienne, Universitt des Sciences et Techniques de Lille, F-59655 Villeneuve d’Ascq, France. 0022-2852/91 $3.00 Copyright 0 I99 I by Academic Press, Inc. AII rights of reproduction in any form reserved.
542
JET SPECTRUM OF THE vj BAND OF OCCCO
543
bending vibrations, and the lowest-lying mode has not yet been located ( 7). To what extent OCCCCCO is quasilinear is not yet determined. The observed spectrum of this species is further complicated by decomposition products. In addition to being of interest due to their dynamics, the cumulenes and related molecules are good candidates for detection in interstellar clouds ( 11); rotational lines attributable to OCCCS have been identified (12, 13), and the species CCC (Z4), CCCCC (15), and CCC0 ( 11, 16) have been definitely identified in interstellar and circumstellar media. The degree of cluttering in a spectrum can be significantly reduced by cooling in order to reduce the effective number of populated states, the challenge being to do so while maintaining sufficient sample in the gas phase. One answer is the use of a supersonic jet in which the internal degrees of freedom of the molecule are cooled by conversion of internal energy to ordered kinetic energy. Cooling to temperatures of a few kelvin has been demonstrated by laser induced fluorescence techniques which were the first spectroscopic methods to be used with such a jet ( 17). Furthermore since the sample is continuously replenished there is no time for breakdown products to form in significant concentration. The first coupling of a supersonic jet to a Nicolet IT-IR spectrometer was reported by Snavely et al. in 1983 (18) with a resolution limited to 0.06 cm-’ . A short time later Dtibal et al. ( 19) published jet spectra recorded with a Bomem interferometer at a maximum quoted resolution of 0.004 cm-’ using a system with external optics. Later the same group built and used successfully a system with a jet inside the Bomem sample chamber ( 20). The use of a supersonic jet for the study of cumulenes posed theoretical and practical questions. It is well known that in a supersonic expansion the vibrational degrees of freedom are not cooled as efficiently as the rotational. As well as a reduction in the extent of the rotational structure a high degree of cooling of the low-lying bending vibrations is required if the density of hot bands in the spectrum is to be significantly reduced. We believed that these low-lying vibrations should be significantly cooled for two reasons. First, there is a well established trend that low-lying vibrations are better cooled than high-lying ones. Second, these degenerate vibrations are coupled to, or one might say, partake of, molecular rotation, especially in the case of quasilinear species such as OCCCO. We thus decided to construct a jet system for use with the Bruker interferometer, keeping in mind applications to molecules of the size of the cumulenes discussed above and to possible unstable species. This paper reports on the design of our system. test spectra obtained with the spectrum of COz, and an experiment with a quasilinear molecule of only moderate stability. OCCCO. For the initial studies of unknown spectra we chose the IQband system of OCCCO, which is the strongest band in the spectrum and falls where the spectrometer is most sensitive. It was also chosen because it lies near the v4 band of OCCCCCO at 22 13 cm-- ’ , as can be seen from Fig. 2a in Ref. ( 9). Despite the difficulties previously discussed, the u3 band system of OCCCO was to a large extent assigned by Fusina ut ul. ( 1) from a room-temperature spectrum recorded with the high resolution laboratory interferometer at the Laboratoire d’Infrarouge at Orsay. They had difficulties assigning lines in the most congested regions, which leaves gaps in the analysis. The simplification achieved with our preliminary experiment using the jet allowed new information to be obtained, complementing the results of the room temperature study.
544
WALTERS ET AL. EXPERIMENTAL
DESIGN DETAILS
We considered carefully a solution analogous to the later Zurich arrangement, placing the jet within the sample chamber of the interferometer system (20). It was decided, however, for several reasons, not to follow such a scheme but instead to construct external optics to transfer the parallel IR beam coming directly from the interferometer optics to the jet. First, the optical axis in the sample chamber of the Bruker instrument lies closer to the ground than that of the Bomem so that there is insufficient space to mount a large diffusion pump. Second, the jet system was to be designed to allow its possible use with millimeter-wave or diode laser spectrometers. Third, the external jet chamber allows for easier introduction of other elements into the experiment such as a multipass cell, or a laser beam for in situ production of unstable molecular species by means of photodecomposition (21) . Figure 1 shows a schematic diagram of the finished system. The overall configuration was determined by the narrow space in the laboratory and along the access path to the interferometer. The system is “portable” on a single cart and can be flanged directly onto the parallel beam output port of the interferometer. Only the roughing pump is mounted separately. A major practical problem is the provision for use with unstable molecules. All connections to the nozzle were made in PFA Teflon and the pressure measured with a piezocrystal gauge. In this way contact of the sample with metal surfaces is minimized. Problems have been reported (22) with the blocking of pulsed nozzles by molecules such as SCCCS owing to cluster formation combined with decomposition. The simple open hole required in any case for the continuously scanning interferometer was expected to be advantageous in this respect. For the initial experiments a metal nozzle was used, but this can later be replaced by a glass nozzle.
spherical
mirror 1’
f=720
i
mm
spherical
mirror
f=280
plane
mirror
(detector plane
_’
-
Bruker
FT-IR
mm
bellows
diffusion
out of
of
drawing)
pump
Spectrometer
FIG. 1. Schematic diagram of molecular jet apparatus and associated optics designed for coupling to the Bruker HR 120 IFS. The nozzle axis is perpendicular to the plane of the figure.
JET SPECTRUM
OF THE v3 BAND OF OCCCO
545
The choice of pump, in particular whether to use a Roots blower or oil diffusion pump, was the subject of considerable initial consideration. For removal of large quantities of gas at above 10e2 mbar a Roots blower is most efficient. An immediate technical problem, however, is to couple the pump to the nozzle chamber without significant loss in pumping speed while avoiding transmission of pump vibrations to the optical system. An undesirable addition to the jet-cooled spectrum is a warm backgroundgas spectrum caused by molecules rethermalized by collisions with the walls of the vacuum chamber. In order to reduce this background spectrum the chamber pressure should be maintained as low as possible. The price and size of Roots blowers with sufficient speed to maintain a pressure below lO-2 mbar with the nozzle open rise steeply as the required pressure is lowered. Contrarily the pumping speed of most diffusion pumps decreases rapidly for pressures exceeding low3 mbar and the pump will soon stall if operated significantly above this limit. The huge physical dimensions and prices of appropriate oil booster pumps make them unsuitable. A 6000-liter set ’ Leybold “Ejector” pump was chosen. This pump spans a pressure range which allows the optimum choice between large sample flow rates and reduced background pressures. It also gives the possibility of switching to a pulsed nozzle system with much lower chamber pressures at a later stage. In the high pressure region this pump requires backing by a pumping system with a speed of at least 200 m3 hr-‘. The combination of an Edwards EHSOOA Roots pump and a E2M80 rotary pump is used to provide the necessary fore-vacuum. For speed of adjustment and in order to protect optical parts during pump warm-up the vacuum chamber can be isolated from the diffusion pump by means of a gate valve. The use of a large diameter (320 mm) valve and special cold-cap rather than traditional baffle results in negligible reduction of the pumping speed. In addition to one port for the nozzle and two ports for the IR beam the vacuum chamber contains five additional vacuum ports through which, for instance, laser radiation can be brought to the jet. Pressure in the chamber is monitored by a Pirani-Penning gauge combination. The parallel IR output beam of the FT-IR spectrometer is focused at the jet by means of a spherical mirror (f = 720 mm) placed at a low angle of incidence ( 11’ ) to limit astigmatism. For a source aperture of 1 mm a focused beam diameter of 2 mm at the jet is obtained with f/10 optics. The diverging beam is then focused on the detector by another spherical mirror (f= 280 mm). The optical path is under vacuum, open to the interferometer vacuum, and pumped separately from the expansion chamber. The IR radiation traverses the chamber through two windows which are only as far from the jet as is necessary to prevent jet disruption hence reducing the problem of a warm background spectrum of scattered molecules. By the use of bellows the window positions can be varied. The nozzle is also adjustable in position along the jet axis, transverse to the IR beam. Allowance has been made to displace the jet 6 cm from the plane of the IR beam in order to accommodate the planned future addition of a multipass optical unit. Two nozzle configurations have been tried so far: a hole nozzle with a diameter of 0.3 mm, and a slit nozzle 10 by 0.01 mm. The measurements reported here were made with a globar source, an InSb detector, an optical filter for the region 1900-2300 cm-’ , and resolution [ 1 / ( maximum optical path difference)] as high as 0.0018 cm-’ . Lower resolution was used for the OCCCO
546
WALTERS ET AL.
spectrum, as is discussed below. The sample of OCCCO was prepared and redestilled from the dehydration of malonic acid with P205. PERFORMANCE
TESTS WITH COz
The system was initialized with measurements of the antisymmetric stretching fundamental band of CO* at 2350 cm-‘. Test measurements were made at various pressures both with undiluted substance and with a seeded argon jet, with different positions of the nozzle relative to the axis of the optical beam, and for both hole and slit nozzles. Figure 2 shows the infrared (naperian) absorbance spectrum of the band with COZ expanded from 500 mbar, undiluted, through the 0.3-mm hole nozzle. Fifty scans
3
al CO,
in nozzle
jet
I
2320
b)
CO2
at room temperature
cl
Jet
spectrum
2330
minus room temperature spectrum
2340
2350
2360
2370
i
2 3f 30
Wavenumber/cm-'
FIG. 2. Spectrum of CO2 expanded from 500 mbar through the 0.3-mm hole nozzle. Fifty scans were taken, each with a S/N ratio of 25 / 1. The three spectra show (a) the raw data in naperian absorbance, (b) the absorption due to room-temperature gas at the pressure (0.001 mbar) of the background in the chamber, and (c) the jet spectrum obtained by subtracting (b) from (a).
JET SPECTRUM
547
OF THE yj BAND OF OCCCO
were taken although the spectrum was easily observable after one scan with a maximum signal-to-noise ratio (S/N) of 25 / 1. The line profile of the R( 0) transition is shown in Fig. 3a to have a width (FWHM) of 0.008 cm-’ dominated by the divergence of the jet parallel to the path of the IR beam. The central dip in the line profile, consistent with previous observations, suggests inhomogeneous cooling across the jet (23). Slight side lobes of the instrumental sine function can be seen to either side of this line, indicating that the resolution employed, 0.004 cm-‘, was insufficient to display the steep flank of the actual lineshape. The line profile of the R( 20) transition shown in Fig. 3b, in contrast, looks like a Doppler-broadened line and has a width of only 0.006 cm-‘, showing no side lobes. The higher J transitions would appear therefore not to originate from the expanding gas. A spectrum was hence taken while effusively leaking COz into the chamber with the diffusion pump running. The chamber pressure was
1
0
Wavenumber/cm-'
d)
R (8)
Cl
1
2.0-
R (81
2.0-
: 6 0 R
+
l.O-
CO.003
cm-'
+
l.O-
0.006
cm-'
4
o.o-
I
o.o-
/
2355.88
I 2355.90
Wavenumber/cm-'
I
L 2355.88
2355.90
Wavenumber/cm-'
FIG. 3. Line profiles of CO2 obtained with the molecular jet: (a) hole nozzle 0.3 mm in diameter, stagnation pressure 500 mbar, resolution 0.004 cm-‘, (b) warm gas in nozzle spectrum, resulting from background pressure of 0.001 mbar, (c) slit nozzle 10 X 0.01 mm, stagnation pressure 90 mbar, resolution 0.0023 cm-‘. (d) slit transverse to infrared beam, stagnation pressure 90 mbar, resolution 0.004 cm-’
548
WALTERS
ET AL.
maintained at the same pressure ( 1 X 1Oe3 mbar) observed when the jet was running. The resulting spectrum of 50 accumulated scans is shown on the same scale in Fig. 2b. The measured linewidth is also 0.006 cm-’ and the high J lines are the same intensity as the high J lines in the jet spectrum so that the origin of the latter is explained. (The Doppler width of these lines at room temperature should be 0.0044 cm-’ ; the observed linewidth corresponds to the convolution of the instrumental lineshape with the Doppler lineshape.) Figure 2c shows the CO;! jet spectrum with the background-gas spectrum directly subtracted; no further manipulation or scaling has been used. Comparison with simulated rotational distributions indicate that the rotational temperature of this spectrum is 2 1 K, although the peak intensity distribution is slightly non-Boltzmann, with the ratio of R( 0) to R( 2) giving a temperature of 19 K and that between R(0) and the higher J lines giving 23 K. The inhomogeneous cooling, mentioned above in connection with the lineshape, would easily account for this range of apparent temperatures. The results shown in Fig. 2 and scans (a) and (b) of Fig. 3 were taken with the center of the focused IR beam 2 mm downstream of the nozzle. With the nozzle closer, strongly non-Boltzmann rotational distribution is seen. With the nozzle further displaced, the rotational temperature in the jet is slightly lower but the warm background observed significantly increases. Spectra were also recorded using a slit nozzle approximately 0.0 1 mm in width and 10.0 mm long, lined up parallel to the IR beam. The area of its opening is roughly equivalent to that of the hole nozzle discussed above. The linewidth was reduced to 0.003 cm-’ as shown in Fig. 3c. The reduction in linewidth shows that the spreading of the jet along the optical axis is significantly reduced. The rotational distribution obtained with a stagnation pressure of 90 mbar (at higher pressures, the signal was so strong that the strongest lines had zero transmittance) could not be matched exactly to any one temperature but shows the greatest similarity to a calculated spectrum for 75 K. When the slit is placed transverse to the IR beam the linewidth, shown in Fig. 3d, increases to 0.006 cm-‘. Interestingly this is still narrower than the linewidth observed with the hole nozzle and the line profile does not show any central dip. The rotational temperature is reduced to 50 K leading to the conclusion that the slit is cooler in the middle than at the ends. Inhomogeneous cooling along the slit is of course consistent with a distribution which is not fully Boltzmann in the spectrum with the slit parallel to the optical axis. The CO2 rotational temperature observed with the slit nozzle parallel to the optical axis is higher than that with the hole nozzle, but for equivalent sample throughput we gain both intensity and resolution, as expected. The optimal nozzle configuration will depend on the sample and the transition intensity to be studied. RESULTS
WITH
OCCCO
The u3 band of OCCCO selected for our first experiment was measured at room temperature, at a resolution of 0.003 cm-’ by Fusina et al. ( I). (The Doppler width is 0.0033 cm-’ .) They were able to assign a large proportion of the observed rovibrational lines between 2000 and 2500 cm-‘. In the less dense sum and difference band regions of this range, the assignments included lines from the sixth excited state of
JET
SPECTRUM
OF
THE
yj BAND
549
OF OCCCO
the u7mode. However, in the dense region of the strong transitions with Av7 = 0, they were not able to identify transitions coming from levels with v7 higher than 2. We recorded this band with the jet system using the hole nozzle. The sample was undiluted, at a stagnation pressure of approximately 250 mbar. The quantity of available sample limited the number of recorded scans to seven, or approximately 15 min total measuring time, so that the spectrum presented here is not typical of the S/N that can be obtained with reasonable measuring times. Because of the double-peaked lineshape discussed above, we recorded the OCCCO spectrum at a resolution of 0.008 cm-‘, which led to single-peaked lines broader than the Doppler width. Since the observed spectrum was not dense, the relatively low resolution of the spectrum was not a disadvantage. Figure 4 shows the overview of a room-temperature spectrum and the jet spectrum, transformed at low resolution. The drastic change in the intensity distribution is clearly seen. Hot bands coming from levels up to v7 = 4, but no higher. can be seen in the jet spectrum. Indeed, even in the low resolution spectrum the Q branches of the hot bands for l,> 1 can be distinguished; these Q branches could not be identified at all in the room-temperature spectrum ( I ) . The observed and assigned bands of the v3 band system are indicated in Fig. 5, which shows the energy levels of the v7 stack in the ground state and with one quantum of u3 excited. The rotational distribution is shown in Fig. 6 for the observed spectrum and a simulation of the fundamental band. This distribution defines a rotational temperature Trot of 60 K. with an uncertainty of +2 K. The simplification of the spectrum is shown in detail in Fig. 7, in which the overlap region between the fundamental and the first hot band is shown for the jet spectrum and a room-temperature spectrum. The new information that was determined from this spectrum, that is, rovibrational
I
-
I
a) :E 1 s
Cell
at
300
K
00
z
2
0.80
+F 0.60
I 2250 Wavenumber/cr.'
FIG 4. Low resolution overview of q fundamental band of OCCCO recorded (a) at room temperature (widely spaced lines at lower wavenumber are CO* impurity) and (b) with the molecular jet system.
550
WALTERS ET AL. 2430 2410
2370
31 33 20 22
FIG. 5. Energy level diagram of the u, vibrational stack of OCCCO in the ground state and with one quantum of the yj mode excited. The bands observed and assigned in the jet spectrum are indicated with vertical lines.
which could not be identified in the analysis of the room-temperature spectrum due to crowding, include the hot bands v3 + 3~: - 313 and v3 + 4v$ - 4v+, the Q branches for the whole series v3 + nv; - nv; for n = 1 to 4, and the low J lines of the first hot band, v3 + Y: - vj. This first hot band is shown in the Loomis-Wood diagram (24) of Fig. 8, and is seen to be strongly shifted by a local resonance affecting both components. The Q branch is displaced by 0.01 cm-’ from the position derived from fitting only the high J lines. The observation of this interaction may help in the identification and interpretation of the set of local resonances observed already for several of the other states in the v3 + 12v7 manifold in the work of Fusina et al. (I ). The line positions for the three hot bands discussed above are given in Tables I and II. The wavenumbers C(J”) for the v3 + 3~: - 3~: and v3 + 4vi: - 4~; bands were fitted to the expression
lines
;( J”) = Ec( J’) - Ec( J”)
(1)
JET SPECTRUM
551
OF THE vj BAND OF OCCCO
0.4 b)
Simulated.
60
K
FIG. 6. Fundamental ~3 band of OCCCO (a) recorded with a molecular jet and (b) simulated for a rotational temperature of 60 K.
with
E,(J) = Gc(v, 1) + B,[J(J + 1) - 12] - D,[J(J
-t 1) - 12]2 + If,[J(J
+ 1) - 1213
(2)
to obtain effective constants for these two subbands. The band origin is given by the difference between the vibrational term values G,( u, I), SO= GC(u’, 1’) - G,.( v”, I”).
(3)
4.0 b) ,”
2
I
1
3.0
D L
2.0
E Q
1.0 no
I 2277
I
I
2278
I
2279
I 2280
Wavenumber/cm-'
FIG. 7. Excerpt of the spectrum of OCCCO in the overlap region of the ~3fundamental and its first hot band from the u, vibrational level, strongly displaced to lower wavenumber. recorded with (a) a molecular jet and (b) at room temperature.
552
WALTERS
ET AL.
l
4
n :r-34,441
Freq.:C
2277.009,
2289.3631
Peaklist
E ZTGSCSl
FIG. 8. Loomis-Wood diagram showing the identification of the low J lines of the e and fcomponents of the first hot band, v, + Y{- Y:, in the spectrum of OCCCO. The weak Q branch is indicated.
This is the same definition of effective constants as that used by Fusina et al. ( 1)) Karyakin et al. (25), and Vander Auwera et al. (4) for the center of each set of ldoublets. For each subband, the lower state constants were held fixed at the effective constants obtained from the results of Vander Auwera et al. (4). Due to the perturbation in the v3 + v$ - v$ hot bands, such a fit is not presented for these subbands; we could not improve on the attempt by Fusina et al. (I) to determine unperturbed constants. The positions of the four Q branches observed and the effective constants for the upper states of the two new subbands are listed in Table III. From the above definitions we find that the Q-branch transition with the lowest possible J value, J = 1, which dominates these weak parallel Q branches, is given by Co = io + (B’ - B”)I.
(4) The calculated and observed Q-branch positions agree within the estimated error of the positions of the Q branches, which are only slightly wider than single lines. The line positions and effective constants obtained for the v3 fundamental and the hot bands for v: = 2’ and vi = 22 agree very well with the data obtained by Fusina et al. (I), and are not repeated here. A rough measure of the vibrational cooling of the v7 excitation could be obtained from the relative intensities of the hot bands and the fundamental band ~3. Assuming that the transition moment is the same for the excitation of one quantum of v3, independent of the excitation of v7, the intensity of the lines for a fixed J in the successive hot bands should reflect the vibrational temperature. Lines for J = 12 and 20 were selected and compared, the HBnl-London factor which depends on I was considered, and the vibrational temperatures listed in Table IV were obtained. The large estimated error for v = 1 = 4 reflects decreasing signal-to-noise ratio of the lines used. The resulting vibrational temperatures appear to decrease with increasing excitation of v7. Since the opposite trend would seem more probable, in view of what is
JET SPECTRUM
OF THE v3 BAND
553
OF OCCCO
TABLE I
Observed Line Positions of the u, = I = 1 Hot Band of vj of OCCCO v = l,l=
v=l,l=l’
J”
I’( J”) /cm-’
1 3
2281.16767
R( J”) /cm-l
J”
P( J”) /cm-’
lf
R( J”) /cm-l
2281.92838
2
2281.31704
2282.088 18
2282.240 49
4
2281.01645
2282.405 03
5
2280.870 50
2282.559 88
6
2280.72178
2282.727 56
7
2280.58102
2282.860 54
8
2280.432 47
2283.058 18
9
2280.272 93
2283.184 27
10
2280.150 56
2283.369 81
11
2279.988 22
2283.509 57
12
2279.850 54
2283.707 23
13
2279.704 75
2283.837 54
14
2279.576 23
2284.047 22
15
2279.42382
2284.168 51
16
2279.304 31
2284.390 72
17
2279.14630
2284.502 82
18
2279.036 06
2284.737 86
19
2278.872 25
2284.840 11
20
2278.77171
2285.089 26
21
2278.60128
2285.180 54
22
2278.51176
2285.444 77
23
2278.333 33
2285.525 17
24
2278.255 89
2285.804 14
25
2278.06908
2285.870 85
26
2278.004 24
2286.167 17
27
2277.807 72
2286.219 70
28
2277.756 81
2286.534 11
29
2277.549 78
2286.573 37
30
2277.513 26
2286.905 31
31
2277.294 61
2286.929 02
32
2277.273 79
2287.28189
33
2277.040 24
2287.28189
34
2277.040 24*
2287.658 68
35
2276.794 02
2287.648 77
36
2276.806 02
2288.040 59
37
2276.547 74
2288.012 93
38
2276.57834
2288.426 23
39
2276.303 18
2268.378 82
40
(2276.353 43)
2288.814 21
41
2276.064 24
2288.748 39
42
2276.133 90
(2289.203 33)
43
2275.826 54
2289.120 39
44
2275.91648
2289.598 24
45
2275.592 87
(2289.492 69)
46
2275.700 34
2289.997 15
47
2275.359 75
2289.87096
48
2275.489 21
2290.395 73
49
2275.130 59
2290.248 09
50
2290.796 17
51
2274.902 52
2290.62791
52
2291.19127
53 55
(2291.01105) 2291.393 36
understood concerning cooling in jets, we conclude that our assumption is not valid, and that the transition moment for one quantum of u3 depends sensitively on the excitation of the quasilinear bending mode u7. From Fig. 5 it can be seen that the pattern of the energy levels in the upper u7 vibrational stack is notably different from the ground state stack. The coupling between v7 and v3 indicated by this difference makes it indeed quite likely that the transition moments of the fundamental v3 band and of the hot bands are different. This may also be the reason that only a few lines of the 2,: = 3 ’ hotband were distinguishable, although they should be stronger than the II: = 44 hot band, as can be seen from Fig. 5. The real vibrational temperature of even the first hot band may therefore be higher than that shown in Table IV: the listed temperatures are probably all lower than the true excitation temperatures.
WALTERS
554
ET AL.
TABLE II
Observed and Calculated Line Positions of the u, = I = 3 and 4 Hot Bands of Y)of OCCCO W =3,1=3 J”
P( J”) /cm-’
3 4
v,=4,1=4 R(Y)
(0 - c) /lo-%m-1
(2267.39033)
/cm-1
(0 - c) /IO-“cm-’
2268.63184
-14
2268.78916
P( 3’) /cm-’
R(J”)
(0 - c) /lo-%n-’
(0 - c)
fcui
/IO-&cm-’
(2262.95741)
21
5
2267.235 10
-181
2268.94582
6
2267.082 82
-107
2269.10469
62
(2261.24082)
(2263.27740)
7
2266.S3095
2269.262 50
29
(2261.08843)
(2263.43829)
a
2266.77961
57
2269.42137
66
2260.93634
-30
2263.59832’
9
2266.62802
81
2269.57976
19
2260.78497
-50
2263.762 59.
10
2266.475 86
11
2269.73877
2
2260.63487
-2
2263.92630*
11
2266.32562
95
(2269.898 22)
2260.48596
103
2264.08637
-132
12
2266.17403
11
2270.05832
2260.33804
249
2264.25006
-141
13
2266.024 14
66
2270.21764
-21
2260.18594
-82
2264.41587
14
2265.87360
29
2270.37737
-51
2260.03732
-124
2264.58016
2259.89040
-55
2259.74385
-5
2264.91165
-31
-50
(2263.11710)
(2261.39382)
38
-145 76 183
5 -57 -1
2264.746 17
15
2265.72337
2
2270.538 15
16
2265.57392
39
(22700.69787)
17
2265.42329
-46
2270.85771
2259.59867
123
2265.07869
-1
18
2265.273 66
-25
2271.01643
-52
2259.45108
-46
2265.24725
150
19
2265.12349
-38
2271.17505
-66
2259.30692
72
2265.41390
57
20
2264.973 17
-30
2271.33378
2259.16132
-9
(2265.58139)
-47
24 10
10
2271.49026
-32
2259.01718
2271.64610
-2
2258.87332
2265.751 W
104
2265.91860
-42
15
2266.08798
-57
72
2266.25864
2
21
2264.82206
22
2264.67083
23
2264.51803
-4
(2271.79995)
2258.73043
24
2264.364 40
40
(2271.95168)
2258.58835
25
2264.20733
-92
2272.10132
45
2258.445 11
-37
26
2264.05165
121
2272.24666
-36
2258.303 76
27
2263.89037
24
2272.38969
2258.16364
28
2263.72666
2258.02275
2
14
2272.52648
-16
-138
-14
-25
-9
2266.59899
-89
95
2266.77127
7
73
2266.94308
15
2257.88392
210
2267.11468
2257.74168
-40
2267.28765
134
2267.459 56
29
2263.56191 2263.38743
31
2263.21486
195
2257.60412
32
2263.03040
-90
2257.46383
-8
2267.63255
2257.32407
-140
(2267.80733)
197
33
10
2266.428 74
30
-143
-53
34
2257.18790
35
(2257.04979)
36
2256.91164
47
-38 7 -92 -118 -172
2267.979 53
38
2268.155 86 -88
5
2268.33007 (2268.50483)
37
109
38
2268.68099
39
(2268.855 20) 53
2269.03127
40
* Lines marked with an asterisk were not used in the fit. TABLE III New Constants Obtained from the u, = I = n Hot Bands of ~3of OCCCO”’ U, = 1,
i&j/cm-’
b,
2281.6229
2
2274.4626
3
2268.0116
2268.009 77 (29)
0.0777016 (31)
16.9 (83)
4
2262.1763
2262.17287 (31)
0.078332 7(10)
49.60 (64)
a) Lower state constants B” = 0.0775043535 B”
=
D’/lO-scm-’
B’jcm-’
&/cm-’
1
0.078027664
b, The listed
were taken from Vader
cm-‘, cm-‘,
D” = 3.22291 D” = 3.19442
values are observed
x lo-’
-0.243 1(59) _
a/10-3cm-’ _ 0.75 0.89
Auwera et al., Ref. (4): for v = 1 = 3, cm-‘,
x 10es cm-‘,
peak positions.
H’/lO-gcm-’
If” = 1.5708 x lo-l3 II” = 1.3862 x lo-l3
cm-‘,
and for v =
cm-‘.
Note that &J = ia + ABI for the lowest J value.
1= 4,
JET SPECTRUM
OF THE v, BAND
OF OCCCO
555
TABLE IV
Vibrational Temperatures Derived from Line Intensities in the Molecular Jet Spectrum of OCCCO Hot band of “3 Ratio of intensity of hot band to 07,/7 ~3 fundamental 0.788 1, 1
Effective81 Tv;b
101 (IO)
2, 2 2, 0
0.430 0.255
79 (5) 64 (5)
3, 3 434
0.160 0.085
63 (5) 70 (20)
%erived
assuming that the transition moments of
hot bands are the same as for the v3 fundamental
CONCLUSIONS
The vibrational temperatures in the jet spectrum of OCCCO shown in Table IV may be considered a lower limit. If we drop the assumption of constant transition moments with excitation of v7, and assume rather that the v7 vibrational stack should show a consistent vibrational excitation temperature, we estimate the vibrational temperature in the v7 bending manifold to be 120 (+20) K. or about 60 K above the rotational temperature, which was 60 K. The degree of cooling obtained with a supersonic jet in the bending manifold of a molecule such as OCCCO, even working with the undiluted sample, is thus amply sufficient to simplify the spectrum drastically while still retaining a large amount of information. The initial results suggest that the system will be very valuable for the study of other cumulenes, or indeed any molecule with a low-lying vibrational mode. Modifications now being implemented include variation in the length of the slit nozzle to optimize sample usage, intensity, and linewidth. It should be possible to obtain significant new information from the bands of OCCCO and the spectra of other cumulenes and other species of a comparable size or complexity. ACKNOWLEDGMENTS
We are grateful to Professor M. Quack and his group for constructive discussion prior to the design of the Giessen system, and Dr. K. M. T. Yamada for discussions concerning design and lineshape. We thank Dr. R. Wilmes for the preparation of a sample of OCCCO. A.D.W. acknowledges a fellowship of the Royal Society. This project was supported by funds from the Deutsche Forschungsgemeinschaft and the Fonds der chemischen Industrie. RECEIVED:
June
10, 1991
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