Coherent raman spectroscopy of HCN complexes

Volume 114, number 1

COHERENT

CHEMICAL PHYSICS LETTERS

RAMAN SPECPROSCOPY

GA. HOPKINS, M. MARONCELLZ, Department

of aemirfry.

15 February 1985

OF HCN COMPLEXES

J.W. NIBLER

Oregon State Univt~sity.

cbrvallis,

OK 97330.

USA

and TR.DYKE Departmenr of Chemistry.

Universiry of Oregon, Eugene. OX 97403.

USA

Received 20 November 1984

Photoacoustic Raman spectroscopy (PARS) is shown to be a sensitive technique for obtaining Raman spectra of hydrogenbonded complexes io the gas phase. Coherent anti-Stokes Raman spectroscopy (CARS) prondes analogous spectra in supersonic expansions. The effective resolution is higher in the latter QSP- due to the cooling and collision-free condit~ons in thelet. The ability to “tune” complex size by changing expansion conditions is demonstrated in the case of HCN.

1. Introduction Our understanding of hydrogen bondmg has grown considerably in the last decade due to gas-phase spectroscopic studies of isolated hydrogen-bonded complexes. The coupling of microwave resonance and supersonic expansion techniques has been particularly successful and has provided equdibrium structures and details of the charge distributions in a variety of simple A-H.. .B model systems [1,2]. However, a more complete description of the hydrogen-bonding potential surface requires vibrational-rotational tiformation. This has been more difficult to obtain although significant progress towards this end has been made recently using infrared laser methods [3,4]. The present work concerns the use of coherent Raman techniques to obtain vibrational spectra of gasphase hydrogen-bonded complexes. While infrared methods have traditionally been employed for this objective [5], Raman spectroscopy is attractive for a number of reasons. First, Raman specha often contain information complementary to that extracted from irifrared studies. Further, the high intensity of the narrow Q branch bands in Raman spectroscopy means that features of complexes are less likely to be masked by the 0 009-26 14/85/S 03 -30 0 Elsevier Science Publishers B.V. (North-Holland Physrcs Publishing Division)

much stronger and extensive monomer P and R bands - a severe problem in the Infrared. Work m this laboratory and elsewhere [6-91 has shown that spectra of molecules in supersonic expansions can be readily obtained with coherent Raman techniques_ As amply demonstrated by microwave [1.2] and electronic [lo] spectroscopies, the extremely low temperatures and collision-free conditions present in such expansions make them ideally suited to studies of weakly bound complexes. Finally, the most interesting lowfrequency hydrogen-bond stretching and bending re@MIS (ml 00 cm-l) are much more accessible by Raman than infrared methods. We report here initial results of studies of homocomplexes of hydrogen cyanide. HCN was chosen as a test case because of its high vapor pressure and because of the simple lmear structure of the HCN dimer [l l] . We have investigated the utility of two coherent Raman methods: photoacoustic Raman spectroscopy (PARS *) for examining static samples and coherent anti-Stokes Raman spectroscopy (CARS) as a probe of complexes formed in supersonic expansions.

’ For a review of PARS, see ref. [ 121. 97

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2. ExperhnentaI In these experiments, a Nd _ YAG laser (Quanta-Ray DCR-IA) provided a primary 532 nm pump beam which consisted of 7 ns pulses at 0.02 cm-l width and 10 Hz repetition rate. Half the ~70 rnI output was used to pump a Quanta-Ray PDL-1 dye laser for the tunable Stokes beam. The latter could be scanned at low resolution (0.3 cm-l) by stepping a grating or at high resolution (0.05 cm-l) by inserting an intracatity etalon and pressure s canning the cavity. For determining Raman shifts, relative frequency calibrations were made using 0.25 and 1 cm-l etalons wlnle HCN monomer lines served as absolute frequency markers. For PARS measurements, the pump and Stokes beams were focused collinearly into a photoacoustic cell with a 300 mm focal length lens. Energies at the sample were ~10 mI per pulse m both beams except at higher sample pressures (>300 Torr) where they were reduced to avoid drelectric breakdown_ The PARS cell [ 131 was a cylindrical aluminum tube 10 cm In length vnth 1” quartz wmdows set at Brewster’s angle. The inner bore of the ceil was tapered to a 10 mm diameter at the center of the cell. The acoustic signal was detected by a Knowles model BT-1759 microphone mounted flush with the inner surface at the celI center. The signal output was filtered (3 dB points at 3 and 30 kHz) end amplified 500-5000X by an Ithaca model 2101 preamplifier. In the CARS experiments, the pump and Stokes beams we;e spatially separated and focused wrth a crossing angle of alo at the jet position This separation greatly reduced the non-resonant contributions from the input lens (300 mm focal length) and windows. The expansion chamber consisted of a 100 mm cubical block with nozzle and pump-out ports and two cylindrical extensions (250 mm) used to reduce laser power densities at the windows. The latter were colored glass filters which served to discriminate further against non-resonant signals. Pump end Stokes energies at the sample were typically between 15 and 30, and 1 and 3 m.I per pulse respectrvely_ After the cell, the beams were recollirnated and the CARS signal isolated with a dichroic mirror and one or more op tical glass filters. It was then sent into a 03 m McPherson monochromator for final rejection of the 532 nm light The signd was detected with en RCA 31032 photomultiplier and the output ampIified 10X 98

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15 February

1985

with a Cornlinear Corp. CLC IO0 video amplifier. The pulsed nozzles used in the jet experiments were BMW fuel injector valves (Bosch). These were modified by soldering a O-25 mm stainless-steel shim onto the injector head and grinding away the excess material to form a conical tip which was then drilled with the desired hole size (0.1 to 025 mm)_ The jet was synchronized with the 10 Hz laser pulses, and dnve electronics were constructed which gave gas bursts of 0.6-l _Orns duration_ The expansion chamber was pumped with a 300 n/rnin rotary pump and a liquid-nitrogen cold trap which were sufficient to maintain the average background pressure at less than 20 mTorr. After imtial preamplification, the data collection and signal processing were the same in PARS and CARS experiments. The respective signals were fitered, time-gated, and further amplified by a Stanford Research model SR-250 gated integrator. The output was then digitized, averaged for lo-50 shots, and stored on a DEC Macro-1 1 computer. Scanning of the dye laser in both high- and low-resolution modes was also computer controlled.

3. Results 3 1. PARS

spectra

of static samples

Fig. 1 shows PARS spectra of the v1 (CN stretch) region of HCN recorded in 15 minutes at 0.3 cm-l resolution. The good S/iV ratio, even for a 10 Torr sample wrthout buffer gas, illustrates the sensitivity of the PARS technique for static gas-phase measurements. HCN complex formation is indicated by the broad shoulder around 2102 cm-l which grows in as the HCN pressure increases. Detailed measurements [15] show a cubic intensity increase at higher pressures, indicating the formation of both dimer and trimer species. Psssuming equal Rarnan cross sections for monomer, dimer, and trimer, the relatrve intendties imply a dirner plus tnrner concentration of 5% at 600 Torr. This number is in good agreement with an independent estimate of 6% based on vapor pressuredensity data [16]. The spectra of fig. 1 also show some of the difficulties with using equibbrium samples in hydrogenbonding studies. First, the mole fraction of dimer and

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Fig. 1. PARS spectra of the CN stretching (~1) region of HCN as a function of pressme. TIie 0 and S branches fiave been sCated up 25x relative to the Q branch. The broad shoulder labeled “D + T” is assignedto unresolveddimer and trimer bands. (Tbe peak at 2093-4 cm-1 is the =r -~a + pa hot band of HCN monomer [14] and not another complex band.)

higher complexes is quite small even at the saturation pressure (650 Torr). Second, stnce hydrogen-bonding molecules usually have large dipole moments (3.0 D in HCN [l l]), pressure broadenmg is a severe problem at these hrgh densities. The monomer 0 and S branch lines (fig. 1) exhibit self-pressurebroadening of 1.8 cm-l/atm, and the broadening of the dimer Qr = 6.5 D [17]) and trimer should be even worse. As a result, no rotational detail is resolved and separate dimer and trimer bands are not dist~~shed_ Thus, while some useful information can be obtained from static samples, the large relative concentrations of complexes and loiv cc&ion frequencies expected in a free jet source make the latter sampling method much more attractive_ 3.2. CARS spectra &

a jke jet

To observe HCN dimer in the jet, it was necessruy regron near the nozzle

to sample in the fidelity

0

1

X/D

2

3

Fig. 2. Expansion conditions at CARS sampling position. (a) Relative dimensions and positioning of the Iase~focus with respect to the nozzle exit. (Neither the focusing of the lasers nor their crossing is noticeable on this scale.) The dashed Lines show 25 K transIational temperature contows calculated for a pure HCN e,xpansion from To = 300 K ir = C$‘Cv = 7/S). @) Relative centerLine temperatures (T/To) and densities @/po) caIcuIated for pure HCN (7 = 7/S, solid curves) and monatomic (7 = 5/3, dashed ctwes) expansions. Jet propertres were calculated from results for ideal rsentroprc expansions given by Gustafson [8].

(X/B = l-3). The expansion used normally consisted of d 0.6 atm of HCN, either neat or rmxed with l-5 atm of Ar, He, or N2 carrier gas. We expect the Jet properties to be approximately those of Ideal isentropIC expansions with heat capacity ratios, y = Cb/GV, between monato~c (513) and diatomic or linear polyatomic (7f5) values. Fig. 2a iliustates the relative drmentions of the expansion and laser beams employed here, atong with tr~slat~on~ tern~~t~e cuntours calculated for a pure HCN jet (r = 7/5; To = 300 K)_ It is clear from fig. 2a that at smah X/O we are sampling a d~t~ution of ‘~e~odyn~c’ states”. In

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fig. 2b we plot the relative temperatures and densities calculated for the centerline of neat HCN (solid curves) and monatomic (dashed curves) expansions. At X/D =

1 S, Tn in the laser focal volume IS about 100 K for neat HCN. ForPo = 0.6 atm, the HCN densities are equivalent to room-temperature samples of S-25 Torr, orders of magnitude higher than achievable with equi-

librium samples at this temperature. The dashed curves show that adding a monatomic carrier gas makes the expansion much colder (7& = 50 K) without changing the HCN density appreciably_ The degree of rotational cooling actually achieved in these expansions is revealed by fig. 3. Trace (a) shows a 0 07 cm-1 resolution scan of the monomer

Ia) Slatlc HCN

15 February 1985

LETI’ERS

v1 Q-branch of a room-temperature static sample at a pressure (1 Torr) such that collision broadening is not important. Comparable linewidths are obtained in the jet (traces (b), (c)) at densities that are much higher, and the collapse of rotational structure due to cooling via supersonic expansion is clearly evident. For the

pure HCN jet (fig. 3b ,X/D = l), we estimate a rotational temperature T,, = 160 k 30 K, reasonably close to the calculated Tu = 130 K. Fig. 3c confirms the additional cooling expected upon addition of argon as a driving gas; we estimate TIot = 90 i 20 K at X/D = 1

(T, = 85 K for a monatomrc expansion). Spectra of hydrogen-bonded complexes in the jet are illustrated by the lower resolution (O-3 cm-l) scans of the v1 region shown in fig. 4_ In pure HCN expansions, fig. 4a, we observe a single, welldefined peak which we attribute to the HCN dimer (D) at +7.8 cm-l with respect to the monomer. This peak revealed no fine structure at 0.07 cm-l resolution_ Comparison

W

(b) Pure

HCN

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Fig. 3 High-resoluhon (0.07 cm-‘)

CARS spectra of the “1 Q branch of HCN flushakng rotational cooling m the jet The static speckurn in (a) was taken at a pressure of 1 TOIT_ The jet spectra of (b) and (c) were remrded at X/D = 1 (D = 0.13 mm). The rotational temperatures (~25 K) were obtained by matching the observed spectra to spectra Qlculated assuming a Boltzma~ distnbution.

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Fig. 4. CARS spectra of tbe y1 region m supersonic expansions showing bands due to complexes_ Peaks are labeled as ?.I = monomer. D = dimer. T = timer and P = higher polymers. AU spectra were remrded under &II&I ex-ption con&tions (To = 300 K, X/D = 1, D = O-1 3 n&n) except for the addition of successively higher pressures of Ar dnving gas (b)-(d)_ In spectrum (d) the region labeled S denotes the position and width of uI in crystalline HCN at 78 K [19]

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with the static PARS spectra of fig. 2 shows the enhanced species discrimination provided by the decreased temperature and pressure in the jet. Assuming again equal monomer and dimer Baman cross sections, we estimate a drmer concentration of lo%, compared to 5% before expansion. .Thus, at X/D = 1, half the dimers have been produced by condensation colhsions during the jet expansion. By altermg the expansion conditions one can produce even more dirners as well as higher aggregates. The spectra of figs. 4b4d show that the effect of adding argon driving gas (at constant HCN pressure) is to increase greatly complex formation in the jet. This results from the lower temperatures obtained for monatomic expansrons (fig. 2b) and from the increase in the number of three-body collisions presumed necessary for complex formation [18]. For the 1 : 2 HCN : Ar mixture shown in fig. 4b, we observe substantially more dimer than in the pure jet. In addition, a new shoulder is observed at +5.3 cm-l from the monomer which we assign to HCN trimer (T). Here the respectIvemoleratiosareroughlyM:D:T=75:15:10. For a 1 : 4 nux (fig. 4c), a new peak is observed at +I.8 cm-l from the monomer and is attributed to higher polymers (P)_ In this case the total CABS signal from HCN complexes is comparable to that from the monomer. Fmally , at 5 5 atm of argon (fig. 4d) the spectrum is completely dominated by the polymer band. Under these expansion conditions, the majority of the HCN is contained in what are probably large solidtie clusters. These results show that it is possible to “tune” the particular complexes observed in jets all the way from dimers to large clusters simply by changing the driving gas pressure. Srmilar trends were seen with N, and He carrier gases. In all cases the spectral features were the same as displayed in fig. 4, indicating that we are indeed observing HCN complexes and not mixed HCNcarrier gas species. We have investigated the effect that a number of other parameters have on complex formation_ For example, mcreasing the nozzle diameter from 0.13 to 025 mm increases complex formation slightly. Although total signal levels were substantially higher in this case, most of the work was done with the smaller diameter nozzle due to the limited pumping speed and in order to conserve sample. Some attempts were made to study spectra as a function of distance from the

15 February 1985

nozzle. Unfortunately, dimer signal levels were insufficient pastX/D = 3 to allow any conclusions regarding the process of compIex formation as a function of jet position. Finally, raising the nozzle temperature by as little as 30°C caused a substantial decrease in the number of higher aggregates formed. This variable thus offers a further means of easily controlling the jet composition.

4. summary We have demonstrated the utility of coherent F&man techniques in studies of simple gas-phase hydrogen-bonded complexes. For the HCN homocomplexes used as a test case, we report the first Rarnan observation of the CN stretching modes of the dimer and trimer spenes. A more detailed account of the spectroscopy of HCN and DCN complexes will be pven elsewhere [ 151. This work is one of the first chemical applications cf PARS spectroscopy and it shows that this method has sensitivity adequate to obtain F&man spectra of small amounts of hydrogenbonded complexes in static room-temperature samples. Unfortunately, at HCN pressures necessary to produce reasonable equilibrium dimer concentrations, pressure broadening is severe and bands due to different complexes are not resolved_ This problem can be overcome by using the non-equilibnum conditions present in supersonic expansions. We have demonstrated that Raman spectra of hydrogen-bonded complexes in free jet expansions can be obtained with CARS spectroscopy_ The higher concentrations of complex species, along with the low temperatures and pressures present in a free jet, allow for much greater resolution than is possible with static samples. The effect of expansion variables such as initial temperature, pressure and composition have been examined. These are shown to offer considerable control over relative and absolute amounts of complexes formed so that one can confidently assign spectral features to dimers and trimers, even in the presence of larger aggregates. Extensions of this work to other spectral regions and other hydrogen-bonded species are underway. Similar experiments using coherent stimulated Barnan gain/loss techniques would be desirable because of the linear scaling with number density and high-resolutron 101

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capab-Sties. At the experimental resolution (0.07 cm-l) in these CARS measurements, no rotational structure was resolved for HCN dimer and trimer species, aithough it was for HCN monomer_ Experiments at higher resolutron could give valuable new information on the structure and lifetimes of vibrational states in simple model systems.

Acknowledgement This research has been supported by the AFOSR and the NSF_ The authors thank Drs. J. Barrett and T. Lundeen for advice and assistance m constructing the PARS cell The pulsed nozzle and drivmg circuitry were based on designs by Dr. H. Selzle and JWN wishes to thank hrm and Professor E. S&lag for their hospitality during a sabbatical at the Technical University of Munich. JWN is also grateful to the Alexander von Humboldt Stiftung for a Senior Scientist Award durmg this period.

References [l] A-C. Lcgon, Ann. Rev. Phys. Chem. 34 (1983) 275. [2] T R. Dyke. in: Topics in curmnt chemistry, Vol. 120, ed_ P. Schuster (S&ger. Berlin, 1984) p_ 85. 131 AS. Pine and WJ. Lafferty, J. Chem. Phys. 78 (1983) 2154; N Ohashi and AS. Pine, J. Chem. Phys 81 (1984) 73; AS. Pine, WJ. Lafferty and B J. Howard, J. Chem. Phys. 81 (1984) 2939.

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[41 MP. Carsassa. CM_ Western. F.G. Celii, DE Brinza and K C. Janda, J. Chem. Phys. 79 (1983) 3227; MJ. Howard, S Burdens& C.F Giese and W-R_ Gentry, J. Chem. Phys. 80 (1984) 4137, and references therein. [51 D J. Millen. J. Mol. Sh-uct 100 (1983) 351; C. Sandorfy, in: Topics in current chernish-y, Vol. 120, ed. P. Schuster (Springer, Berho, 1984) p_ 41, and references therem. 161 P_ Huber-Walchli, DM. GuthaJs and J-W_ Nobler. Chem. Phys. Letters 67 (1979) 233; P. Huber-WaJchh and J-W Nibler, J. Chem. Phys. 76 (1982) 273. 171 F_ Koenig, P. Oesterlin and R L. Byer. Chem. Phys. Letters 88 (1982) 477; E.K. Gustafson, J.C. McDanieland RL. Byes, Opt. Letters 7 (1982) 434_ 181 EX. Gustafson, PhD. Thesis, Stanford University (1983). and A. Owyoung, Chem. Phys. 191 JJ. Valentine, P. Werick Letters 75 (1980) 590 31 (1980) 197. r101 D.H. Levy,Ann. Rev.Phys.Chem. 1111 L.W. Buxton, EJ. Campbell and W.H. Flygare. Cl-rem. Phys. 56 (1981) 399. iI21 JJ. Barrett, in: Chemtcal apphcations of nordincar Raman spectioscopy, ed. A.B. Harvey (Academic Press, New York, 1981) p. 89_ 1131 T-F. Lundeen, PhD_ Thesis, Oregon State Umversity (1984). 1141 J. Bendsten and H GM. Edwards, J. F&man Spcco-y. 2 (1974) 407. [ISI M. Maroncelli, G.A. Hopkins, J.W Nibler and T-R. Dyke. to be published. [16] W.F. Giauque and RA. Ruebrwien, J. Am. Chem. Sot. 61 (1939) 2626. [17] EJ. Campbell and S-G. Kukolich, Chem. Phys. 76 (1983) 225. [ 181 D. Coulomb, RE. Good and RF. Brown, J. Chem. Phys. 52 (1970) 1545; TA. Mime. AE. Vandegsift and F.T. Greene, J. Chem. Phys. 52 (1970) 1552. [19] M. Pezolet and R. Savoie, Can. J. Chem. 47 (1969) 3041_