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Vibrational predissociation of OCS clusters excited near the ν3 vibration of the monomer
Volume 160, number 5,6
CHEMICAL
PHYSICS LETTERS
25 August 1989
VIBRATIONAL PREDISSOCIATION OF OCS CLUSTERS EXCITED NEAR THE v, VIBRATION OF THE MONOMER R. DUREN,
G. HILLRICHS,
M. LIESNER and S. MOHR
Max-Planck-lnstitut ftir Striimungsforschung, D-3400 GSttrngen, Federal Republic of Germany Received 2 1March 1989; in final form 15 June 1989
OCS clusters have been produced in a supersonic beam. Dissociation spectra for these clusters have been measured with a diode laser and a bolometer detector. Two sets of results are presented: ( 1 ) Results for large clusters with discrete sampling through a range of 80 cm-’ around the Y, monomer absorption. In this group the size distribution ofthe clusters is varied with the stagnation pressure in the source. Broad spectra are obtained (between 20 and 40 cm-’ depending on the pressure) with a maximum blueshift of 12 cm- ’ from the monomer absorption. (2) Results for small clusters with a continuous scan. Here narrow ( = 50 MHz) dissociation lines are observed, if the beam parameters are chosen so as to produce dimers preferentially.
1. Introduction Infrared predissociation spectroscopy has been used to study the properties of van der Waals clusters over a wide range of sizes. This method was introduced by Stoles and coworkers [ 1 ] and applied subsequently in many laboratories [ 2,3 ] _The main goals of such studies are to obtain insight into the structure of the clusters in terms of ordered, disordered, crystalline or liquid aggregation. The characteristic feature of such measurements is that the dissociation process relies on the interaction of the components of the clusters with each other. Hence these measurements are related to line broadening experiments in cells. Another window to the cluster configuration is opened by high resolution measurements yielding narrow line dissociation. As a general rule such spectra occur preferentially for small clusters [ 4-61. In contrast to the aforementioned measurements these spectra are characteristic of a global excitation of the complex, which is then considered as a well defined molecule with specific states. Consequently the information gained from these measurements refers to “stable” structures of the clusters with specific excited state lifetimes. The experimental technique is to produce the clusters in a supersonic expansion with a carrier gas, let the beam interact with light from a laser and detect 602
the dissociation with a bolometer. By varying the beam conditions (its pressure and composition) the size and the temperature of the clusters are varied. In this publication we present experimental results for high resolution predissociation of OCS clusters seeded in a He beam and excited with a diode laser near the v3 vibration (2062 cm- ’ ) of the OCS monomer. Two sets of results are given, one with emphasis on the overall shape of the dissociation spectrum with discrete, wide step, sampling and a second one with continuous sampling. They are associated with different beam conditions, the first section referring to large clusters showing no fine structure; the second one referring mainly to dimers showing narrow dissociation lines. Compared with similar clusters of linear triatomic molecules such as N20 [ 7 ] and CO, [ 5 1, which have been investigated in some detail, little is known about the OCS system. Gentry and coworkers [8] presented photodissociation spectra of OCS dimers and trimers, excited near the first overtone of the v2 bending mode. In addition there is a substantial amount of work referring to the line broadening of infrared absorption in cells [ 91. The main advantage of our study is the use of a diode laser which, in contrast to the discrete line lasers often used in this type of experiment, is capable of a continuous wide range scan of the spectra. To-
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CHEMICAL
5,6
PHYSICS LETTERS
gether with the narrow spectral width of the laser this gives us the possibility to determine narrow structures in the dissociation spectra with high accuracy.
2. Experiment A schematic view of our apparatus (which is similar to that of ref. [ 1] ) is shown in fig. 1. Briefly, a 2% or a 10% mixture of OCS in He is expanded into the vacuum through a 30 pm nozzle at stagnation pressures of up to 20 bar. In a second differentially pumped chamber, separated from the first by a 900 urn skimmer positioned 20 mm away from the nozzle the laser beam interaction takes place. In the third chamber the molecular beam hits the diamond absorber (2.5 X 2.5 mm’) of a silicon bolometer, operated at = 1.8 K. The bolometer detector can be moved off axis so that the molecular beam can pass into a fourth chamber to a detector with a quadrupole mass spectrometer. The laser system is equipped with several lead salt diodes to cover the spectral range between 2005 and 2 100 cm-‘. The operating temperature of the diodes ranges from 20 to 50 K. Normally laser radiation oc-
Monochromator
Fig. 1. Schematic view of the apparatus. 0: molecular beam source, BD: bolometer detector, MP: multiple pass cell and D, to D,: infrared detectors.
25 August I989
curs in several modes with a typical multimode power of 1 mW. To select a single mode of the laser spectrum an external monochromator is used. Usually one mode can be scanned continuously through a range of 1 cm-’ before a mode hop occurs. To reduce instabilities of the laser frequency we stabilized the laser for the high resolution scans with a tunable Fabry Perot etalon as described in ref. [ lo]. For absolute frequency calibration a fraction of the laser beam passes through a gas cell containing a few Torr of OCS. Additional relative frequency markers are obtained from a fixed length confocal Fabry Perot etalon with a free spectral range of 300 MHz. To enhance the interaction of the laser beam with the molecular beam a multiple pass cell is employed. It consists of two gold coated parallel mirrors on both sides of the molecular beam. It provides about 15 crossing of the laser beam and the molecular beam under an angle of approximately 85 ‘. The multiple pass cell is placed at a position along the beam to yield flight times of 250 us between the interaction zone and the bolometer. This value defines the upper limit of any dissociation lifetime which can be determined with this apparatus. Internal excitation of the molecules in the beam results in an increased flow of energy to the bolometer if the lifetime of the excited state is longer than the time of flight to the bolometer. Excitation of the clusters leading to their dissociation within the time of flight to the bolometer will decrease the energy flow, if the fragments are scattered out of the angle of acceptance of the bolometer, which is 0.6”. Some considerations about the spectral resolution of our experimental apparatus should be reported. The use of multiple pass optics leads to a non-orthogonal crossing between the laser and molecular beam. This introduces a Doppler shift Au= (v/i,) X sin 6, where 0 denotes the deviation from perpendicular crossing. The velocity u of the He-seeded OCS molecules and clusters is taken to be 1400 m/s, leading to a shift of 25 MHz. The spread of the velocity distribution Au is estimated to be IO%, resulting in a Doppler broadening 6u= (Au/&,) sin 0 of the absorption line of 2.5 MHz. Strictly speaking, the small divergence of the molecular beam and also of the slightly focused laser beam increases the Doppler broadening. Therefore it seems realistic to assume a broadening due to the Doppler effect of c7 MHz. 603
Volume 160, number 5,6
CHEMICAL
PHYSICS LETTERS
25 August 1989
The broadening due to the short flight time of the molecules through the laser spot is estimated to be 2 MHz. Power broadening also appears to be small with our laser conditions and should be less than 1 MHz. Additionally, the stabilisation of the laser requires a frequency modulation of a few MHz, which also means a broadening of the laser line. As shown later we found that the width of a monomer transition was about 40 MHz. This value is taken as the spectral resolution and is the result of convolution of the broadening mechanisms discussed above and a main contribution from the frequency jitter of the laser mode.
3. Results
for large clusters
Fig. 2 shows a series of results - the dissociation signal as a function of the wavelength - for various stagnation pressures ranging from 3.2 to 20 bar. The spectra show only dissociation of clusters. For moderate pressures, i.e. smaller clusters, we find a broad absorption band with a maximum blue-shifted by 12 cm-’ from the monomer gas phase absorption and a shoulder red-shifted from it. With higher pressures an additional broad red-shifted and unstructured absorption appears. Note in particular that no absorption is observed at the position of the OCS crystal positioned at 2004 cm-’ [ 111, which means that the clusters have not adopted a crystalline structure. It should be noted that these spectra are measured only at the discrete points shown in the graph, and not as a continuous scan. However, in addition to these measurements we have taken high resolution scans through a range of 1 cm-’ around many of the discrete sampling points. The results of these scans do not show the narrow lines reported below for other beam conditions. The variation of the beam parameters in this series changes the size distribution and the temperature of the clusters. Although our apparatus is equipped with a mass spectrometer we do not have reliable estimates as to the size of the clusters due to the well known problems with fragmentation in the ion source [ 121 and the limited mass range of our quadrupole. We are therefore restricted to coarse estimates deduced by analogy from other measurements [ 13,14 1. These lead to magnitudes of up to 10 or 100 OCS 604
2020.0
204&Y
wavelength
2060.0
2080.0
/ cm-’
Fig. 2. This dissociation spectrum of large OCS clusters, seeded in a beam of He, for various stagnation pressures in the source. The concentration of OCS in He is 10%. The solid line represents a least-squares fit to the data with two Gaussians.
molecules per cluster for the various beam conditions. For a stagnation pressure of 3.2 bar we have measured a rotational temperature of 3 K for the lowest J states of the OCS monomers. Under the given conditions this value may be taken as a lower limit for the temperature of the clusters. This is in agreement with the measurements of Jucks et al. for similar CO1 dimers under comparable beam conditions [IS]. To interpret the spectra quantitatively we used a model developed in refs. [ l&l6 1. This model takes only the resonant dipole-dipole interaction into account [ 31. In ref. [ 151 this model was used to explain the spectra of CO* clusters. The success of this investigation encouraged us to apply this model, with some modifications, to OCS. The authors illustrate the importance of the resonant interaction by com-
Volume 160, number 5,6
CHEMICAL PHYSICS LETTERS
paring the shift and the width of the ‘*COz u3 absorption peak with that of the 13C02 peak. The latter appears to be very narrow and nearly unshifted while the former is shifted by 20 cm ’ from the solid absorption and has a width of about 10 cm-‘. For a 13COz molecule the nearest neighbour is normally a ‘*CO2 molecule, so that resonant interaction is not possible, while the V~absorption of the “COZ molecule is strongly affected by resonant interaction with a neighbouring 12C02 molecule. Furthermore the model is based on the assumption that the dissociation spectrum of a large cluster is determined by the interaction of two neighbouring monomers within the cluster, which are treated as a pair of coupled oscillators. The energy of this system is given by
where p, and pz are the transition moments of the individual molecules and i,, is the unit vector from one molecule to the other. For this distance r12 and a known orientation of the molecules, AE is the displacement of the absorption with respect to the monomer line. Examination of eq. ( 1) shows that there is do energy shift for a T-shaped configuration. If the two moments are perpendicular to ilZ and parallel or antiparallel to each other, one obtains a shift of i 13.5 cm-’ (taking ,u,=p2=0.34 D [17] and r12=3.5 A). In a collinear configuration the shift would be f 27 cm-‘. In the calculation of the spectrum we included the restraint that the two projections p, of p, onto the laboratory z axis, describing the laser polarisation, must have the same sign. Other configurations will not contribute to the dissociation spectrum. In a simulation of the measured spectra we select the vectors of c,, ,I+ and r,* at random and calculate the displacement according to eq. ( 1). For this displacement the contribution to the spectrum is given by the sum of pf weighted with a geometrical and the Boltzmann factor. In this simulation the potential function is introduced via the Boltzmann factor. A full potential, valid especially for the intermediate distances occurring in the cluster molecule,, is not available in the literature. Therefore we applied a spherically symmetric LennardJones ( 12, 6) model with t = 232 cm - 1 and R, = 3.5 A [ 181. A geometrical restraint, namely that A6 is smaller than some limiting angle of the order
25 August 1989
of 45 ’ (i.e. preferentially planar configuration), was introduced in the Monte Carlo selection of the simulation. The parameters of the potential are deduced from those given in ref. [ 19 1. The well depth is taken to be equal to the one quoted there, the simulation not reacting sensitively to variations of this value. The equilibrium distance required a change to yield agreement with our measurements. Fig. 3 shows the result from the simulation with T=20 K and Aqt 45’ together with the experimental points for 5.0 bar. The agreement with this measurement is quite reasonable and beyond this particular case the measurements at 3.2 and at 11 bar are similarly well described with these parameters. Hence we conclude that the structure and the temperature of the clusters up to 11 bar of pressure are reproduced by the model. The main feature of the spectrum, the blue-shifted peak, is well reproduced with its displacement of 12 cm-’ from the monomer absorption. The analysis of the simulation reveals that this maximum is mainly due to contributions from preferentially parallel or antiparallel orientations of the molecules. The model fails to describe the dissociation spectra measured at the highest stagnation pressures. We consider that under these con-
2035.0
2080.0
wavelength
2085.0
/ cm-’
Fig. 3. Experimental (open circles) and simulated (solid line) dissociation spectra of large OCS clusters for a stagnation prcssure of 5 bar.
605
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CHEMICAL PHYSICS LETTERS
ditions the clusters become so large and the internal temperature so high that the assumption of only pairwise interactions is no longer valid. At the same time additional geometrical restrictions may become important. The agreement between this simple simulation and experiment for the lower pressures suggests that the OCS cluster spectrum is dominated by the resonant dipole-dipole interaction. One reason for this surprisingly good agreement may be the large transition moment (0.34 D) of the vr vibration. This may be compared with the change of the permanent dipole moment of OCS due to excitation of the uj vibration, which is z 0.026 D [ 171. This can only be responsible for a change of the interaction energy of less than 0.16 cm-‘, for the intermolecular distance of 3.5 A. The conclusions which can be drawn from this simulation may be summarized in the following way. The dissociation of large clusters up to a certain size is determined by the interaction of pairs of molecules embedded in the cluster. Their geometrical arrangement appears to be such that a planar parallel or antiparallel configuration prevails. For this geometrical arrangement of the “active pair” of molecules in the cluster results from other sources for the dimer give additional support. Microwave and radiofrequency spectroscopy as well as molecular beam deflection point in this direction [ 20,2 1 ] as does the infrared photodissociation reported earlier [ 8 1. In this latter reference it was concluded that the two OCS molecules are located at identical sites which is again consistent with a parallel or antiparallel dimer configuration. Finally, it may be noted that this type of structure is also found for similar dimers of other linear molecules such as CO2 [5,22] and NrO [23,24]. One interesting question, which arises from these conclusions, is whether this arrangement is preferred as a property of the cluster or because the absorption dissociation process is favoured for this geometry. We raise this point with some emphasis since the latter argument would seriously restrict the strength of these experiments in terms of the determination of the potentials in cases such as the one studied here.
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4. Results for small clusters To obtain the results which will be reported in this section several changes were necessary compared with those of the previous section. Experimentally, the slow single mode scanning facility described above had to be introduced. Moreover the beam conditions needed to be changed, in particular the concentration was reduced to 2O/bof OCS in the He beam. At this concentration the built up of small clusters is preferred - we find only dimers in the mass spectrometer - and more efficient cooling will occur. Fig. 4 shows one section of the measured spectra as an example. In the lowest trace of the graph narrow dissociation lines of about 50 MHz halfwidth are clearly seen (shown as upward peaks) separated from well defined monomer excitation lines (shown as downward peaks). With this spectrum the calibration line from the reference cell (uppermost trace) and the calibration from the fixed length FPl are given. These lines are definitely single mode scans with the individual lines being established by about 30 points in the computer controlled scan. Obviously from these data the width of the individual lines is established with reliable accuracy (see below). At the sharp edge of the spectrum at the right, obtained by blocking the laser, the remaining broad band background of the dissociation establishes the noise of the data. Taking this into account the spectrum is seen to be comparably “rich” with all of the lines well reproducable. Looking closer at the data one finds the monomer lines to be narrower than the dissociation lines. We use the former lines to determine the spectral resolution of our apparatus, which can be represented very well by a Gaussian profile of 45 MHz fwhm. Fitting Voigt profiles of which the Gaussian part is kept fixed to the dissociation lines we determine the lifetime of these states from the Lorentzian part. The common result of these fits to numerous lines of the spectrum is a width of 3.1 MHz (fwhm) for the Lorentzian leading to a dissociation lifetime of r= 52 + 7 ns. This evaluation is valid with the assumption of homogeneous broadening of the lines observed. Considering the high resolution of our measurements and the close similarity of the result with those for N@ [ 41 and CO2 [ 51 this assumption seems plausible. But as long as it remains unverified
Volume 160, number 5,6
CHEMICAL
PHYSICS LETTERS
2072.89
25 August 1989
2072.84
wavelength
2072.79
/ en;-’
Fig. 4. Lowest trace: High resolution, single mode scan of small OCS clusters with 2% of OCS seeded in a beam of He with a stagnation pressure of 6 bar. Dissociation lines are shown as upward peaks, excitation lines of the monomer as downward peaks. Upper trace: Relative wavelength markers with 300 MHz separation. Top trace: Cell absorption of OCS for absolute calibration.
by calculation of the spectra, our value should strictly speaking be considered as a lower limit. (As mentioned above the upper limit is given by the flight time to the bolometer of 250 ps.) This result differs substantially from the excitation of OCS dimers with 2uz photons (bending mode) for which a lifetime of 1.45 ps has been reported [ 8 1. The corresponding stronger coupling of the bending mode to the van der Waals bond appears a plausible additional hint to the planar/parallel configuration of the dimer. Work is in progress to fit the high-resolution spectra and to obtain accurate information about the rotational constants and the structure of the dimer.
Acknowledgement We wish to thank D. Bassi, M. Zen and J.P. Bouanich for valuable discussions. The use of the computer facilities of the Gesellschaft fur wissen-
schaftliche Datenverarbeitung acknowledged.
in
Gijttingen
is
References r1 ] T.E. Gough, R.E. Miller and G. Stoles, J. Chem. Phys. 69 (1978)
1588.
12 R.E. Miller, 5. Phys. Chem. 90 (1986) 3301. [3 I. Geraedts, S. Stolte and J. Reuss, Z. Phys. A 304 (1982) 167. [4 1R.E. Miller and R.O. Watts, Chem. Phys. Letters 105 (1984) 409. [5 I K.W. Jucks, Z.S. Huang, R.E. Miller, G.T. Fraser, A.S. Pine and W.J. Lafferty, J. Chem. Phys. 88 (1988) 2185. [6 1G.D. Hayman, J. Hodge, B.J. Howard, J.S. Muenter and T.R. Dyke, Chem. Phys. Letters 118 (1985) 12. M. Gauthier, J. Chem. Phys. 88 (1988) 5439. t; M.A. Hoffbauer, K. Liu, C.F. Giese and W.R. Gentry, J. Phys. Chem. 87 (1983) 2096. [9] J.P. Bouanich, G. Blanquet, J. Walrand and C.P. Courtoy, J. Quant. Spectry. Radiative Transfer 36 ( 1986) 295. [IO ] M. Reich, R. Schieder, H.J. Clar and G. Winnewisser, Appl. Opt. 25 ( 1986) 130.
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[ ii] F.D. Verderame and E.R. Nixon, J. Cbem. Phys. 44 (1966) 43. [ 121 U. Buckand H. Meyer, Phys. Rev. Letters 52 (1984) 109. [ 131 R.E. Miller, R.O. Wattsand A. Ding, Chem. Phys. 83 ( 1984) 155. [ 14) T.E. Cough, R.E. Miller and G. Stoles, J. Phys. Chem. 85 (1981) 4041. [ 151 J.A. BamesandT.E. Gough,J.Chem.Phys. 86 (1987) 6012. [ 161 W. Breymann and R.M. Pick, J. Chem. Phys. 84 (1986) 4187. [ 171 A. Foord and D.H. Whiffen: Mol. Phys. 26 ( 1973) 959. [ 181J.O. Hirschfelder, C.F. Curtiss and R.B. Bird, Molecular theory of gases and liquids (Wiley, New York, 1954).
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[ 191 R.P. Leavitt, J. Chem. Phys. 72 ( 1980) 3472. [ 20 ] J.M. Lobue, J.K. Rice and SE. Novick, Chem. Phys. Letters 112 (1984) 376.
[ 211 SE. Novick, R.D. Suenram and F.J. Iovas, J. Chem. Phys. 88 (1988) 687. [22]M.A. Walsh, T.H. England, T.R. Dyke and B.J. Howard, Chem. Phys. Letters 142 (1987) 265. [ 23 ] B.J. Howard, XIth International Symposium on Molecular Beams, ed. M.A.D. Fluendy, University of Edinburgh, (1987). [24] Z.S.HuangandR.E. Miller, J. Chem. Phys. 89 (1988) 5408.
CHEMICAL
PHYSICS LETTERS
25 August 1989
VIBRATIONAL PREDISSOCIATION OF OCS CLUSTERS EXCITED NEAR THE v, VIBRATION OF THE MONOMER R. DUREN,
G. HILLRICHS,
M. LIESNER and S. MOHR
Max-Planck-lnstitut ftir Striimungsforschung, D-3400 GSttrngen, Federal Republic of Germany Received 2 1March 1989; in final form 15 June 1989
OCS clusters have been produced in a supersonic beam. Dissociation spectra for these clusters have been measured with a diode laser and a bolometer detector. Two sets of results are presented: ( 1 ) Results for large clusters with discrete sampling through a range of 80 cm-’ around the Y, monomer absorption. In this group the size distribution ofthe clusters is varied with the stagnation pressure in the source. Broad spectra are obtained (between 20 and 40 cm-’ depending on the pressure) with a maximum blueshift of 12 cm- ’ from the monomer absorption. (2) Results for small clusters with a continuous scan. Here narrow ( = 50 MHz) dissociation lines are observed, if the beam parameters are chosen so as to produce dimers preferentially.
1. Introduction Infrared predissociation spectroscopy has been used to study the properties of van der Waals clusters over a wide range of sizes. This method was introduced by Stoles and coworkers [ 1 ] and applied subsequently in many laboratories [ 2,3 ] _The main goals of such studies are to obtain insight into the structure of the clusters in terms of ordered, disordered, crystalline or liquid aggregation. The characteristic feature of such measurements is that the dissociation process relies on the interaction of the components of the clusters with each other. Hence these measurements are related to line broadening experiments in cells. Another window to the cluster configuration is opened by high resolution measurements yielding narrow line dissociation. As a general rule such spectra occur preferentially for small clusters [ 4-61. In contrast to the aforementioned measurements these spectra are characteristic of a global excitation of the complex, which is then considered as a well defined molecule with specific states. Consequently the information gained from these measurements refers to “stable” structures of the clusters with specific excited state lifetimes. The experimental technique is to produce the clusters in a supersonic expansion with a carrier gas, let the beam interact with light from a laser and detect 602
the dissociation with a bolometer. By varying the beam conditions (its pressure and composition) the size and the temperature of the clusters are varied. In this publication we present experimental results for high resolution predissociation of OCS clusters seeded in a He beam and excited with a diode laser near the v3 vibration (2062 cm- ’ ) of the OCS monomer. Two sets of results are given, one with emphasis on the overall shape of the dissociation spectrum with discrete, wide step, sampling and a second one with continuous sampling. They are associated with different beam conditions, the first section referring to large clusters showing no fine structure; the second one referring mainly to dimers showing narrow dissociation lines. Compared with similar clusters of linear triatomic molecules such as N20 [ 7 ] and CO, [ 5 1, which have been investigated in some detail, little is known about the OCS system. Gentry and coworkers [8] presented photodissociation spectra of OCS dimers and trimers, excited near the first overtone of the v2 bending mode. In addition there is a substantial amount of work referring to the line broadening of infrared absorption in cells [ 91. The main advantage of our study is the use of a diode laser which, in contrast to the discrete line lasers often used in this type of experiment, is capable of a continuous wide range scan of the spectra. To-
0 009-2614/89/$ ( North-Holland
03.50 0 Elsevier Science Publishers Physics Publishing Division )
B.V.
Volume 160, number
CHEMICAL
5,6
PHYSICS LETTERS
gether with the narrow spectral width of the laser this gives us the possibility to determine narrow structures in the dissociation spectra with high accuracy.
2. Experiment A schematic view of our apparatus (which is similar to that of ref. [ 1] ) is shown in fig. 1. Briefly, a 2% or a 10% mixture of OCS in He is expanded into the vacuum through a 30 pm nozzle at stagnation pressures of up to 20 bar. In a second differentially pumped chamber, separated from the first by a 900 urn skimmer positioned 20 mm away from the nozzle the laser beam interaction takes place. In the third chamber the molecular beam hits the diamond absorber (2.5 X 2.5 mm’) of a silicon bolometer, operated at = 1.8 K. The bolometer detector can be moved off axis so that the molecular beam can pass into a fourth chamber to a detector with a quadrupole mass spectrometer. The laser system is equipped with several lead salt diodes to cover the spectral range between 2005 and 2 100 cm-‘. The operating temperature of the diodes ranges from 20 to 50 K. Normally laser radiation oc-
Monochromator
Fig. 1. Schematic view of the apparatus. 0: molecular beam source, BD: bolometer detector, MP: multiple pass cell and D, to D,: infrared detectors.
25 August I989
curs in several modes with a typical multimode power of 1 mW. To select a single mode of the laser spectrum an external monochromator is used. Usually one mode can be scanned continuously through a range of 1 cm-’ before a mode hop occurs. To reduce instabilities of the laser frequency we stabilized the laser for the high resolution scans with a tunable Fabry Perot etalon as described in ref. [ lo]. For absolute frequency calibration a fraction of the laser beam passes through a gas cell containing a few Torr of OCS. Additional relative frequency markers are obtained from a fixed length confocal Fabry Perot etalon with a free spectral range of 300 MHz. To enhance the interaction of the laser beam with the molecular beam a multiple pass cell is employed. It consists of two gold coated parallel mirrors on both sides of the molecular beam. It provides about 15 crossing of the laser beam and the molecular beam under an angle of approximately 85 ‘. The multiple pass cell is placed at a position along the beam to yield flight times of 250 us between the interaction zone and the bolometer. This value defines the upper limit of any dissociation lifetime which can be determined with this apparatus. Internal excitation of the molecules in the beam results in an increased flow of energy to the bolometer if the lifetime of the excited state is longer than the time of flight to the bolometer. Excitation of the clusters leading to their dissociation within the time of flight to the bolometer will decrease the energy flow, if the fragments are scattered out of the angle of acceptance of the bolometer, which is 0.6”. Some considerations about the spectral resolution of our experimental apparatus should be reported. The use of multiple pass optics leads to a non-orthogonal crossing between the laser and molecular beam. This introduces a Doppler shift Au= (v/i,) X sin 6, where 0 denotes the deviation from perpendicular crossing. The velocity u of the He-seeded OCS molecules and clusters is taken to be 1400 m/s, leading to a shift of 25 MHz. The spread of the velocity distribution Au is estimated to be IO%, resulting in a Doppler broadening 6u= (Au/&,) sin 0 of the absorption line of 2.5 MHz. Strictly speaking, the small divergence of the molecular beam and also of the slightly focused laser beam increases the Doppler broadening. Therefore it seems realistic to assume a broadening due to the Doppler effect of c7 MHz. 603
Volume 160, number 5,6
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PHYSICS LETTERS
25 August 1989
The broadening due to the short flight time of the molecules through the laser spot is estimated to be 2 MHz. Power broadening also appears to be small with our laser conditions and should be less than 1 MHz. Additionally, the stabilisation of the laser requires a frequency modulation of a few MHz, which also means a broadening of the laser line. As shown later we found that the width of a monomer transition was about 40 MHz. This value is taken as the spectral resolution and is the result of convolution of the broadening mechanisms discussed above and a main contribution from the frequency jitter of the laser mode.
3. Results
for large clusters
Fig. 2 shows a series of results - the dissociation signal as a function of the wavelength - for various stagnation pressures ranging from 3.2 to 20 bar. The spectra show only dissociation of clusters. For moderate pressures, i.e. smaller clusters, we find a broad absorption band with a maximum blue-shifted by 12 cm-’ from the monomer gas phase absorption and a shoulder red-shifted from it. With higher pressures an additional broad red-shifted and unstructured absorption appears. Note in particular that no absorption is observed at the position of the OCS crystal positioned at 2004 cm-’ [ 111, which means that the clusters have not adopted a crystalline structure. It should be noted that these spectra are measured only at the discrete points shown in the graph, and not as a continuous scan. However, in addition to these measurements we have taken high resolution scans through a range of 1 cm-’ around many of the discrete sampling points. The results of these scans do not show the narrow lines reported below for other beam conditions. The variation of the beam parameters in this series changes the size distribution and the temperature of the clusters. Although our apparatus is equipped with a mass spectrometer we do not have reliable estimates as to the size of the clusters due to the well known problems with fragmentation in the ion source [ 121 and the limited mass range of our quadrupole. We are therefore restricted to coarse estimates deduced by analogy from other measurements [ 13,14 1. These lead to magnitudes of up to 10 or 100 OCS 604
2020.0
204&Y
wavelength
2060.0
2080.0
/ cm-’
Fig. 2. This dissociation spectrum of large OCS clusters, seeded in a beam of He, for various stagnation pressures in the source. The concentration of OCS in He is 10%. The solid line represents a least-squares fit to the data with two Gaussians.
molecules per cluster for the various beam conditions. For a stagnation pressure of 3.2 bar we have measured a rotational temperature of 3 K for the lowest J states of the OCS monomers. Under the given conditions this value may be taken as a lower limit for the temperature of the clusters. This is in agreement with the measurements of Jucks et al. for similar CO1 dimers under comparable beam conditions [IS]. To interpret the spectra quantitatively we used a model developed in refs. [ l&l6 1. This model takes only the resonant dipole-dipole interaction into account [ 31. In ref. [ 151 this model was used to explain the spectra of CO* clusters. The success of this investigation encouraged us to apply this model, with some modifications, to OCS. The authors illustrate the importance of the resonant interaction by com-
Volume 160, number 5,6
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paring the shift and the width of the ‘*COz u3 absorption peak with that of the 13C02 peak. The latter appears to be very narrow and nearly unshifted while the former is shifted by 20 cm ’ from the solid absorption and has a width of about 10 cm-‘. For a 13COz molecule the nearest neighbour is normally a ‘*CO2 molecule, so that resonant interaction is not possible, while the V~absorption of the “COZ molecule is strongly affected by resonant interaction with a neighbouring 12C02 molecule. Furthermore the model is based on the assumption that the dissociation spectrum of a large cluster is determined by the interaction of two neighbouring monomers within the cluster, which are treated as a pair of coupled oscillators. The energy of this system is given by
where p, and pz are the transition moments of the individual molecules and i,, is the unit vector from one molecule to the other. For this distance r12 and a known orientation of the molecules, AE is the displacement of the absorption with respect to the monomer line. Examination of eq. ( 1) shows that there is do energy shift for a T-shaped configuration. If the two moments are perpendicular to ilZ and parallel or antiparallel to each other, one obtains a shift of i 13.5 cm-’ (taking ,u,=p2=0.34 D [17] and r12=3.5 A). In a collinear configuration the shift would be f 27 cm-‘. In the calculation of the spectrum we included the restraint that the two projections p, of p, onto the laboratory z axis, describing the laser polarisation, must have the same sign. Other configurations will not contribute to the dissociation spectrum. In a simulation of the measured spectra we select the vectors of c,, ,I+ and r,* at random and calculate the displacement according to eq. ( 1). For this displacement the contribution to the spectrum is given by the sum of pf weighted with a geometrical and the Boltzmann factor. In this simulation the potential function is introduced via the Boltzmann factor. A full potential, valid especially for the intermediate distances occurring in the cluster molecule,, is not available in the literature. Therefore we applied a spherically symmetric LennardJones ( 12, 6) model with t = 232 cm - 1 and R, = 3.5 A [ 181. A geometrical restraint, namely that A6 is smaller than some limiting angle of the order
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of 45 ’ (i.e. preferentially planar configuration), was introduced in the Monte Carlo selection of the simulation. The parameters of the potential are deduced from those given in ref. [ 19 1. The well depth is taken to be equal to the one quoted there, the simulation not reacting sensitively to variations of this value. The equilibrium distance required a change to yield agreement with our measurements. Fig. 3 shows the result from the simulation with T=20 K and Aqt 45’ together with the experimental points for 5.0 bar. The agreement with this measurement is quite reasonable and beyond this particular case the measurements at 3.2 and at 11 bar are similarly well described with these parameters. Hence we conclude that the structure and the temperature of the clusters up to 11 bar of pressure are reproduced by the model. The main feature of the spectrum, the blue-shifted peak, is well reproduced with its displacement of 12 cm-’ from the monomer absorption. The analysis of the simulation reveals that this maximum is mainly due to contributions from preferentially parallel or antiparallel orientations of the molecules. The model fails to describe the dissociation spectra measured at the highest stagnation pressures. We consider that under these con-
2035.0
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Fig. 3. Experimental (open circles) and simulated (solid line) dissociation spectra of large OCS clusters for a stagnation prcssure of 5 bar.
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ditions the clusters become so large and the internal temperature so high that the assumption of only pairwise interactions is no longer valid. At the same time additional geometrical restrictions may become important. The agreement between this simple simulation and experiment for the lower pressures suggests that the OCS cluster spectrum is dominated by the resonant dipole-dipole interaction. One reason for this surprisingly good agreement may be the large transition moment (0.34 D) of the vr vibration. This may be compared with the change of the permanent dipole moment of OCS due to excitation of the uj vibration, which is z 0.026 D [ 171. This can only be responsible for a change of the interaction energy of less than 0.16 cm-‘, for the intermolecular distance of 3.5 A. The conclusions which can be drawn from this simulation may be summarized in the following way. The dissociation of large clusters up to a certain size is determined by the interaction of pairs of molecules embedded in the cluster. Their geometrical arrangement appears to be such that a planar parallel or antiparallel configuration prevails. For this geometrical arrangement of the “active pair” of molecules in the cluster results from other sources for the dimer give additional support. Microwave and radiofrequency spectroscopy as well as molecular beam deflection point in this direction [ 20,2 1 ] as does the infrared photodissociation reported earlier [ 8 1. In this latter reference it was concluded that the two OCS molecules are located at identical sites which is again consistent with a parallel or antiparallel dimer configuration. Finally, it may be noted that this type of structure is also found for similar dimers of other linear molecules such as CO2 [5,22] and NrO [23,24]. One interesting question, which arises from these conclusions, is whether this arrangement is preferred as a property of the cluster or because the absorption dissociation process is favoured for this geometry. We raise this point with some emphasis since the latter argument would seriously restrict the strength of these experiments in terms of the determination of the potentials in cases such as the one studied here.
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4. Results for small clusters To obtain the results which will be reported in this section several changes were necessary compared with those of the previous section. Experimentally, the slow single mode scanning facility described above had to be introduced. Moreover the beam conditions needed to be changed, in particular the concentration was reduced to 2O/bof OCS in the He beam. At this concentration the built up of small clusters is preferred - we find only dimers in the mass spectrometer - and more efficient cooling will occur. Fig. 4 shows one section of the measured spectra as an example. In the lowest trace of the graph narrow dissociation lines of about 50 MHz halfwidth are clearly seen (shown as upward peaks) separated from well defined monomer excitation lines (shown as downward peaks). With this spectrum the calibration line from the reference cell (uppermost trace) and the calibration from the fixed length FPl are given. These lines are definitely single mode scans with the individual lines being established by about 30 points in the computer controlled scan. Obviously from these data the width of the individual lines is established with reliable accuracy (see below). At the sharp edge of the spectrum at the right, obtained by blocking the laser, the remaining broad band background of the dissociation establishes the noise of the data. Taking this into account the spectrum is seen to be comparably “rich” with all of the lines well reproducable. Looking closer at the data one finds the monomer lines to be narrower than the dissociation lines. We use the former lines to determine the spectral resolution of our apparatus, which can be represented very well by a Gaussian profile of 45 MHz fwhm. Fitting Voigt profiles of which the Gaussian part is kept fixed to the dissociation lines we determine the lifetime of these states from the Lorentzian part. The common result of these fits to numerous lines of the spectrum is a width of 3.1 MHz (fwhm) for the Lorentzian leading to a dissociation lifetime of r= 52 + 7 ns. This evaluation is valid with the assumption of homogeneous broadening of the lines observed. Considering the high resolution of our measurements and the close similarity of the result with those for N@ [ 41 and CO2 [ 51 this assumption seems plausible. But as long as it remains unverified
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wavelength
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Fig. 4. Lowest trace: High resolution, single mode scan of small OCS clusters with 2% of OCS seeded in a beam of He with a stagnation pressure of 6 bar. Dissociation lines are shown as upward peaks, excitation lines of the monomer as downward peaks. Upper trace: Relative wavelength markers with 300 MHz separation. Top trace: Cell absorption of OCS for absolute calibration.
by calculation of the spectra, our value should strictly speaking be considered as a lower limit. (As mentioned above the upper limit is given by the flight time to the bolometer of 250 ps.) This result differs substantially from the excitation of OCS dimers with 2uz photons (bending mode) for which a lifetime of 1.45 ps has been reported [ 8 1. The corresponding stronger coupling of the bending mode to the van der Waals bond appears a plausible additional hint to the planar/parallel configuration of the dimer. Work is in progress to fit the high-resolution spectra and to obtain accurate information about the rotational constants and the structure of the dimer.
Acknowledgement We wish to thank D. Bassi, M. Zen and J.P. Bouanich for valuable discussions. The use of the computer facilities of the Gesellschaft fur wissen-
schaftliche Datenverarbeitung acknowledged.
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[ 211 SE. Novick, R.D. Suenram and F.J. Iovas, J. Chem. Phys. 88 (1988) 687. [22]M.A. Walsh, T.H. England, T.R. Dyke and B.J. Howard, Chem. Phys. Letters 142 (1987) 265. [ 23 ] B.J. Howard, XIth International Symposium on Molecular Beams, ed. M.A.D. Fluendy, University of Edinburgh, (1987). [24] Z.S.HuangandR.E. Miller, J. Chem. Phys. 89 (1988) 5408.