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Pulsed discharge source of supersonically cooled transient species
Chemical Physics North-Holland
I55 ( 1991) 267-274
Pulsed discharge source of supersonically cooled transient species R.
Schlachta, G. Lask
Institut ftir Physikalische und Theoretische Chemie der TU Miinchen. W-8046 Garching, Germany.
S.H. Tsay Department of Chemistry, The Ohio Bate University, Columbus, OH, 43210, USA
and V.E. Bondybey Institut fir Physikalische und Theoretische Chemie der TV Miinchen, W-8046 Garching, Germany and Department of Chemistry, The Ohio State University, Columbus, Ohio, 43210, USA Received 9 March I99 1
A source combining dc discharge with pulsed supersonic expansion is shown to be a versatile tool for spectroscopic studies of a variety of transient species. High concentrations of cold small radicals have been generated using suitable precursors, and detected by laser induced fluorescence. Discharging of halogenated methanes provides an efficient source of halogenated carbenes. Molecular radical cations are similarly efftciently produced, probably as a result of the Penning ionization process in collisions with metastable rare-gas atoms. Reactive sputtering of electrodes is shown to result in the production of metal halides and similar simple metal compounds. An application of the discharge source as a useful tool for matrix isolation studies is proposed and discussed.
1. Introduction Molecular ions [ l-31, free radicals [ 4-71, and other reactive, transient species [ 8,9] are important intermediates in many chemical reactions, and their properties are of considerable interest. Because of their high reactivity, however, they are usually quite short-lived, and their study represented a nontrivial experimental challenge. Traditionally, different types of electric discharges proved to be useful sources of a variety of transient species [ 7,10-201. Species formed in the discharge are usually rotationally, vibrationally and electronically quite hot, and this further complicates their spectroscopy. To improve signal to noise, and simplify the spectra, it is of course convenient to cool the species, and one way to accomplish this is matrix isolation spectroscopy. The combination of gaseous microwave or electric discharge with the matrix technique proved 0301-0104/91/%
to be a very successful combination [ 15,19 1. A novel, elegant technique, combining a corona discharge with supersonic jets was recently developed by Engelking [ 201. In his technique, a Pt electrode held at a potential of some 15 kV is located behind the orifice of a continuous Pyrex nozzle. A discharge in the expanding gas results in the formation and production of the reactive intermediates, which are cooled by adiabatic expansion. In some applications, pulsed experiments have considerable advantages over continuous systems 1211. Sustaining a pulsed supersonic expansion requires smaller and less expensive pumping systems, moreover pulsed laser systems are cheaper, more flexible, and more easily tunable over wide spectral ranges. Furthermore, one can obtain additional information about lifetimes, time evolution and dynamics, not readily obtainable in a cw laser experiment.
03.50 0 1991 - Elsevier Science Publishers B.V. (North-Holland)
268
R. Schlachta et al. / Pulsed dischargesource of transient species
In our laboratory we have been interested for some years in developing a pulsed nozzle discharge source, and the purpose of the present manuscript is to describe some of our efforts in this direction. We have designed an extremely simple source combining electric discharge with a pulsed supersonic expansion. No sophisticated power supplies or control1 circuits are required, since the opening and closing of the valve itself serves to turn on and off the discharge. Early in the development of this source several years ago our intent was to apply it to gas phase studies of Ar-OH, Ar-CN and similar van der Waals complexes, which we had previously observed in raregas matrices. These species were, in the meantime, observed and studied in several other laboratories. The free radical and ion source has, however, many other potential applications, both for the gas phase work and for low-temperature matrix studies. Here we describe our pulsed source, discuss some of its advantages, and describe several initial applications to studies of various types of supersonically cooled reaction intermediates.
R3
Fig. 1. A simplified schematic of the dc discharge control circuit. The H.V. power supply charges the capacitor C. The currents in the charging and discharging parts of the circuit are controlled by the current limiting resistors RI and R2. The opening of the valve V introduces the gaseous sample into the channel in the teflon fixture T mounted on the valve, and results in a current flowing between the metal electrodes. The operation of the valve can be monitored and the current measured by displaying the voltage drop over a small resistor R3 on the oscilloscope screen.
2. Experimental details A mixture of a suitable precursor in 3-4 bar argon expands through a pulsed valve and expands through a teflon discharge fixture mounted on the bottom of the valve. In most experiments a modified electromagnetic fuel-injection valve (Bosch) is used, although several experiments were performed with a Lasertechnics piezoelectric valve. In the presence of gas in the flow channel an electrical discharge takes place between two metal electrodes located in the low channel, as shown in fig. 1. The electrodes were made from aluminium or copper in the present experiments, but other metals could of course also be used. A voltage of typically 1000 V dc is applied at all times. A simple current circuit consisting of capacitors and current limiting resistors is used to stabilize the discharge. The resistors were chosen so as to make the time constant t = RC of the loading circuit small compared to the time between two consecutive gas pulses. During the experiment the time dependence and stability of the discharge were monitored by an oscilloscope as shown in fig. 2. After opening the electromagnetic valve, the current rises abruptly (fig.
Fig. 2. Typical current patterns observed in the apparatus in fig. 1. Using a Lasertechnics piezoelectric valve (a) or an electromagnetically driven fuel injection valve (c ) . The more complicated pattern in fig. la is due to bouncing of the valve. Fig. 2b shows the optical emission intensity from the discharge flame for the pulse of fig. 2a monitored by a photomultiplier. See text.
2~). It then decays in an exponential fashion for the duration of the gas pulse, and falls off again sharply after the valve closes. The time profile of the pulse thus provides a useful diagnostic of the operation of the valve and shows that the pulses are usually between 0.8-3 ms long, depending on the shape and duration of the driving electric pulse, The working frequency, limited by our excimer laser, is typically 5 Hz. Bouncing of the valve was a difficulty which we have experienced mainly with the piezoelectric Lasertechnics device. The problem could be reduced by adjusting the operating conditions, but we were unable to eliminate it completely. This can be seen in the fig. 2a, where the current profile suggests that for each trigger pulse the valve opens and closes twice. The reactive species expand further on through a small orifice from the discharge fixture into a vacuum chamber evacuated by a 500 a/s roots blower. The discharge naturally produces ions, electrons, radicals, and electronically excited species, and results in a visible “flame” issuing from the orifice. Further information about the operation of the discharge can be obtained by monitoring this visible emission by a photomultiplier. This is shown for the piezoelectric valve pulse of fig. 2a in the panel 2b. The time resolved profile of the emission resembles that of the discharge current, but is slightly shifted in time by several microseconds needed for the discharge products to move from the discharge through the orifice into the region monitored by the photomultiplier. It can also be noted that the emission intensity decays somewhat more rapidly than the discharge current. This may be due to the depletion of the parent species. The species present in the “flame” are probed by the pulses of an excimer (EMG 500, Lambda Physik) pumped dye laser ( 1.3 mJ per pulse, 14 ns pulse duration, FL 2000, Lambda Physik ). In some experiments, a nitrogen laser pumped dye laser was used (Molectron ) . The laser-induced fluorescence is collected by a quartz lens system and detected by a photomultiplier tube (R928, Hamamatsu) and digitized in a waveform recorder (2262, LeCroy ). The signal is then averaged and processed by a HP QS20 computer controlling the experiment. Laser excitation spectra are recorded by monitoring the undispemed fluorescence intensity as a function of the laser wave-
h
Photodiode NlM-Trigger Wavefom-Rec.
t+ Fig. 3. A timingdiagramof the experiment.The openingof the electrom~~eticvalveintroducesgasinto the discharge channel and fires the discharge. The dye laser is fired with a suitable delay after the valve is opened, and the laser induced fluorescence is digitized in a waveform recorder. The laser light is also detected by a diode, used in conjunction with a NIM delay unit to stop the digitization process, and initiate the data transfer into a computer.
length. A schematic diagram of the timing of the valve pulse, the discharge pulse, dye-laser pulse, the laserinduced fluorescence, and the detector signal is shown in fig. 3. Absolute wavenumber calibration of the dye-laser scanning system was accomplished by recording independently the spectra of the CN free radical produced from acetonitrile, and comparing the observed lines with well-known literature data. 3. Results and discussion 3. I. Production ofsimple free radicals A variety of diatomic free radicals were produced with high efficiency. For instance, intense OH, NH, CN and C2 spectra were produced by employing H20, NH, CH3CN and C2H2 as precursors. In general, the spectra exhibited rotational temperatures of 5-50 K, depending on the expansion parameters, but in many cases vibrational temperatures of several thousand Kelvin. In order to characterize the operation of the valve better, we have performed a series of experiments with aniline in the discharge, in order to be able to monitor not only the products, but also the precursor. Fig. 4a shows a typical time profile of the parent aniline fluorescence in an experiment using the pie-
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R. Schlachta
et al. / Pulsed discharge source qftransient
species
i 0
I’
LOO
800
1200
0 DELAY
LOO
800
I200
1600
lpsi
Fig. 4. Typical patterns observed by scanning the delay of the dye laser with respect to the valve opening and monitoring the fluorescence intensity. The double appearance of the signals is due to bouncing of the valve. (a) The parent aniline fluorescence; (b) the NH fragment; (c ) CN radical; (d ) C, radical.
zoelectric valve, produced by scanning the delay between the opening of the valve and the probe laser pulse. The bouncing of the Lasertechnics valve is again shown by the double appearance of the time resolved profile. When the discharge is turned on, the aniline fluorescence intensity decreases to less than one half of its initial value as a result of its decomposition, and at the same time the spectra of several products can be observed. Fig. 4b shows the time profile for NH. This is undoubtedly a primary reaction product, probably resulting from a reaction of the parent with rare-gas metastable atoms or ions. It exhibits a time profile resembling that of the parent. In addition, several other products were observed, most notably the CN and C) radicals, suggesting a fairly extensive decomposition of the aniline precursor. These products, however, exhibit time profiles considerably different from that of aniline itself, and of the NH primary product. This is probably due to the fact that they are higher order products whose production requires multiple collisions. They are therefore probably preferentially formed on the leading or trailing edge of
the pulses, where more turbulence sions can be expected.
and more colli-
3.2. Spectra of halocarbenes Fluorochlorocarbons and their dissociation products play an important part in the chemistry of the upper atmosphere and in the destruction of stratospheric ozone [ 6,22 1. In spite of the importance of the halocarbenes and other halocarbon dissociation products for atmospheric chemistry and as chemical intermediates, the spectroscopic information available about their properties and spectroscopy is still fragmentary [ 23 1. The rotationally resolved gas phase spectrum of CF2 was studied by Mathews [ 241. A number of other halocarbenes were studied by lowtemperature matrix techniques. More recently, a highresolution electronic spectrum of CCIZ was reported [ 25 1, and both CF2 and CC& were studied by microwave spectroscopy [ 26,271. Much less information is, however, available about the unsymmetrically substituted halocarbenes, CXY. In our discharge system described above, the dihalocarbenes are very efficiently produced from hal-
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R. Schlachta et al. / Pulsed discharge source of transient species
ogenated methane precursors and other halogenated organic compounds. For instance, excellent quality CCIZspectra were produced from Ccl,. An example of such a spectrum is shown in fig. 5. While the CC& properties are relatively well known [25,28], the technique is equally well applicable to other carbenes, for which spectral information is lacking. Work in this direction is currently in progress. 3.3. Production of molecular ions Above we proposed that the major mechanism of the formation of the radical fragments is the reaction of the precursor molecules with metastable rare-gas atoms. It is well documented that reactions of metastable atoms with many organic molecules result in the production of the corresponding radical cations via Penning ionization. We have tested this aspect by adding precursors whose ionization yields fluorescent molecular ions. For instance, the addition of pentafluorobenzene to the gaseous flow resulted in a visibly greenish discoloration of the discharge “flame”. Fig. 6 shows the result of scanning the dye laser and observing the resulting fluorescence. A rather strong,
I 24500
‘.
I
24600
I
24700 WAVENUMBER
24800
.
I
.
24900
[cm-‘]
Fig. 6. A short section of the fluorescence excitation spectrum of the pentatluorobenxene radical cation CsF5H+ about 1500-2000 cm-’ above the origin. As discussed in the text, the radicals are probably produced by reactions of the metastable rare-gas atoms with the parent compounds.
structured spectrum is observed, whose comparison with the published data leaves little doubt that it is due to the pentafluorobenzene radical cation [293 11. It should be noted that the region scanned in the present work corresponds to energies some 15002000 cm-’ above the origin band, where the FranckCondon factors are very weak and where the spectrum is rather congested. It is clear that excellent spectra could be produced in the region near the electronic origin, where the Franck-Condon factors are favorable. There is no spectral congestion and the bands are intense and well-resolved. 3.4. Reactive sputtering of metal atoms and spectra of simple metal compounds
m
19000
19400
19800
WAVENUMBER
20200 [cm-‘]
Fig. 5. An example of a fluorescence excitation spectrum of CClr produced by discharging about 0.1% Ccl, in argon. The spectrum consists of a series of “Fermi polyads” due to interaction between the progression of the symmetric Y, stretch with overtones of the bending frequency vr as previously reported on the basis of rare-gas matrix studies in ref. [ 281 and recently confirmed in the beautiful gas phase experiments by Clouthier and co-workers (see ref. [25] ).
The impact of ionized atoms or other reactive intermediates on the surface of the electrode may dislodge the metal atoms. This may, on the one hand, be used to investigate the reactive sputtering process as a function of the discharge parameters, such as pressure, voltage, current, nature of the carrier gas, or nature of the reactive species. On the other hand, it may serve as a valuable spectroscopic tool for studying e.g. metal oxides, sulfides, halides and other high-temperature molecules. As an example, fig. 7 exhibits the gas phase spec-
R. Schlachta
212
et al. / Pulsed discharge source qf transient
specie3
previously. We find lifetimes of 320 rt 30 ns for the B+-X and 25Orfi30 ns for the C-X transition (see table 1). The techniques reported here should be quite generally applicable, and should permit a wide variety of rotationally cold halides, nitrides, oxides, carbides and similar species to be conveniently studied.
3 m =: H
3.5. Pulsed di~~char~e as a source o~tra~sie~t species of matrix isolation studies
z
W iZ f-4
L 22300
22700
23500
23100
WAVENUMBER
23900
[cm“l
Fig. 7. Spectrum showing the B ‘n-+X ‘C+ and C ‘Z+-+X ‘I+ transitions of CuBr generated by entraining about 0.1% of CFBr3 in argon flowing through the discharge and by reactive sputtering of the electrodes by the fragments produced in the discharge. The individual bands are labeied by a letter denoting the upper electronic state, and by a subscript and a superscript giving the vibrational quantum numbers U”and IJ’. Table I Molecular constants of CuBr State
r, (cm-‘)
& (cm-‘)
WJ, (cm-‘)
X B c
0 23045 23461
313.99 283.64 293.84
1.16
1.46 1.15
T (ns) 320+30 205230
trum of CuBr, obtained by entraining CF2Br2 or CFBr, in the carrier gas flowing through the discharge. Comparison with literature data [ 321 shows unequivocally that they are due to the B+-X and CtX transitions of diatomic CuBr. The obtained spectroscopic constants are given in table 1. It can be noted that similar to the other species studied, the CuBr molecules are vibrationally very hot, and levels up to V”= 5 are detected. The strong CuBr spectrum, in fact, interfered with our efforts to record and analyze the spectrum of the brom~~uor~r~ne, and for this reason we had to switch to aluminum electrodes. The spectroscopic constants of the CuBr molecule are, of course, relatively well known [ 33 3. However, as a by-product, time resolved fluorescence also provides information about the lifetimes of the observed species, and the lifetimes of CuBr were not measured
The source described above should prove to be a very convenient and efficient source of molecular ions, free radicals and other transient species for matrix isolation studies, too, and this was an additional motivation for its development. Discharges, in particular those using microwave cavities, have been frequently used in matrix studies [ 34-371. In a source of this nature, the mixture of the matrix gas with a suitable precursor flows through a 10 mm diameter quartz tube located in the microwave discharge cavity, at a pressure in the order of 0.1 Torr, and is eventually expanded through a 0.5-l mm diameter nozzle into the vacuum enclosure and deposited on the cold substrate. Under these circumstances one can easily calculate that the average precursor will remain in the discharge region for some 200 ms. If the decomposition occurs in the middle of this time period, on average, then the products will undergo some 104-lo5 collisions with other molecules and 500 wall collisions prior to being expanded through the pinhole and deposited on the cryogenic substrate. Therefore, there are many possibilities of undesired secondary recombinations or neutralization reactions. In order to circumvent this problem, alternative geometries were designed where, for instance, the pure matrix gas is discharged and mixed only shortly before the deposition and secondary reactions can be suppressed by such geometries, the extent of mixing and the conversion efficiencies can be relatively small. In the pulsed discharge source described here, on the other hand, the parent molecules are in intimate contact with the carrier gas during the discharge, but are in the nearly cotlision free jet environment a few microseconds later. After less than 1 ms, they are embedded in the solid matrix. Consequently, the conversion efficiencies can be high, but the opportunities for secondary reactions are minimized.
R. Schlachta
et al. / Pulsed discharge source of transient
In our gas phase studies with the discharge source, the typical current was 5 mA with a pulse duration of 1 ms. If one assumes that electrons are the primary current carriers, then one can calculate that approximately 5 x 1O- ’ ’ mol of electrons are needed. In such a pulse, about 2.5 umol of the gaseous mixture are deposited. Assuming a typical matrix dilution of 1: 1000, this corresponds to 2.5 nmol or 2.5 X lop9 mol of the precursor compound. If only one molecule of the precursor had to be broken up to produce each charge carrier, one could deduce that about 2% of the precursor molecules would be broken up. Actually, breaking a chemical bond typically requires much less energy (300-500 kJ/mol) than ionization ( z 1000 kJ/mol ) , and undoubtedly many more molecules are fragmented than are ionized. In the experiment using an aniline precursor, whose concentration can be conveniently monitored by laser induced fluorescence, we found that when the discharge was turned on, the parent signal dropped to about 30% of the original value, this means that about 702 of the precursor molecules were broken up. It should be noted that in the gas phase fluorescence experiments the current was held at about 4-5 mA. While at low currents the discharge was well localized between the electrodes, inside the discharge fixture and only relatively little light from the discharge could reach the detector, when it was increased beyond this value, the character of the discharge suddenly changed. The glow discharge extended far into the vacuum chamber and produced a bright luminescent “flame”. This emission, which was of course bothersome in the gas phase LIF studies, would not be a problem in matrix work. Consequently, if a more complete decomposition of the sample were desired, the currents could easily be increased by at least one additional order of magnitude. Similar to the pulsed laser vaporization source we recently described, the pulsed discharge should also permit a much quicker generation of transient species in sufficient concentrations for spectroscopic characterization. Operating the source at only 2 pps, as in the above experiments, will result in deposits in the order of 2-5 mmol in about 10 min, whereas in conventional continuous deposition experiments several hours are usually required. An additional advantage of the pulsed operation is that unlike continuous deposition it provides clear, unscattering de-
species
273
posits of excellent optical quality even with heavier matrix gases such as argon or krypton. The pulsed discharge unit of the type described here should therefore provide an extremely useful source of transient species for matrix isolation studies, providing the advantages of a more complete precursor conversion and much less opportunity for secondary reactions. We are currently in the process of building a matrix setup which should soon allow us to test the source in matrix experiments.
4. Summary A very simple dc discharge source for spectroscopic studies of supersonically cooled ions, radicals and other transient species has been constructed, and its operation and applications have been described. We have shown that, using suitable precursor molecules, extremely high concentrations of cold reactive intermediates can be produced and detected by time resolved pulsed laser induced fluorescence. Various halocarbenes can be produced by discharging halogenated methanes. The Penning ionization process by the rare-gas metastables produces molecular radical cations similarly efficiently. Simple metal halides can be produced in high yields by reactive sputtering of the metal electrodes, and by using suitable reactants the technique can be applied to produce other simple metal compounds. A potential application of the discharge source as an efficient tool for matrix isolation studies is proposed and discussed.
Acknowledgements This work was supported in part by the National Science Foundation under grant no. CHE8803169 and by The Ohio State University Research Grant Program. Assistance by the Fonds der Chemischen Industrie is also gratefully acknowledged.
References [ 1 ] J.P. Maier, Ion and Cluster Ion Spectroscopy and Structure (Elsevier, Amsterdam, 1989).
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R. Schlachta et al. / Pulsed discharge source of transient species
[2] A. Carrington and B.A. Thrush, The Spectroscopy of Molecular Ions (Cambridge University Press, Cambridge, 1988). [ 3 ] T.A. Miller and E. Bondybey, Molecular Ions: Spectroscopy, Structure and Chemistry (North-Holland, Amsterdam, 1983). [ 41 W.A. Pryor, Frontiers of Free Radical Chemistry (Academic Press, New York, 1980). [ 51 J.K. Kochi, Free Radicals (Wiley, New York, 1973). [6] J.G. Anderson, Ann. Rev. Phys. Chem. 38 (1987) 489. [ 71 G. Her&erg, The Spectra and Structure of Simple Free Radicals (Cornell University Press, Ithaca, 197 1). [ 81 N.S. Jsaacs, Reactive Intermediates in Organic Chemistry (Wiley, London, 1974). [9] D.H. Russell, Gas Phase Inorganic Chemistry (Plenum Press, New York, 1989). [ lo] E.L. Leasure, C.R. Mueller and T.Y. Ridley, Rev. Sci. Instr. 46 (1975) 635. [ 111 J.Q. Seamy, Rev. Sci. Instr. 45 ( 1974) 589. [ 121 S. Sharpe and P. Johnson, Chem. Phys. Letters 107 ( 1984) 35. [ 13lT.H. Allik, B.B. Brady, G.W. Flynn and G.B. Spector, J. Phys. Chem. 88 ( 1984) 3204. [ 141 M. Yamashita and J.B. Fenn, J. Phys. Chem. 88 (1984) 4451. [ 151 M.E. Jacox, J. Phys. Chem. 88 ( 1984) 445. [ 161 A.T. Forrester, Large Ion Beams (Wiley, New York, 1988). [ 171 J.M. Cook, T.A. Miller and V.E. Bondybey, J. Chem. Phys. 68 (1978) 2001. [ 18 ] J.M. Cook, T.A. Miller and V.E. Bondybey, J. Chem. Phys. 69 (1978) 2562.
[ 191 V.E. Bondybey and J.H. English, J. Chem. Phys. 67 ( 1977) 3405. [ 201 P.C. Engelking, Rev. Sci. Instr. 57 ( 1986) 2274. [ 2 1 ] G. Stoles, Atomic and Molecular Beam Methods (Oxford University Press, Oxford, 1988 ) . [22] R.S. Stolarski, Sci. Am. 258 (1988) 20. [23] M.E. Jacox, J. Phys. Chem. Ref. Data 17 ( 1988) 269. [24] C.W. Mathews, Can. J. Phys. 45 (1967) 2355. [ 251 D.J. Clouthier and J. Karolczak, J. Chem. Phys. 94 ( 199 1) [26] k_X. Powell and D.R. Lide, J. Chem. Phys. 45 ( 1966) 1067. [ 27 ] M. Fujjtake and Eizi Hirota, J. Chem. Phys. 9 1 ( 1989) 3426. [28] V.E. Bondybey, J. Mol. Spectry. 64 (1977) 180. [29] M. Heaven, T.A. Miller and V.E. Bondybey, J. Chem. Phys. 76 (1982) 3831. [30] V.E. Bondybey and T.A. Miller, J. Chem. Phys. 70 ( 1979) 138. [ 3 I] V.E. Bondybey, T.A. Miller and J.H. English, J. Am. Chem. Sot. 101 (1979) 1248. [32] Ritschl, Z. Physik 42 (1927) 172. [33] K.P. Huber and G. Hetzberg, Molecular Spectra and Molecular Structure, Vol. 4 (Van Nostrand Reinhold, New York, 1979). [34] F.O. Rice and M. Freamo, J. Am. Chem. Sot. 75 ( 1953) 548. [ 35 ] M. MC Catty Jr., G.W. Robinson, J. Am. Chem. Sot. 8 1 ( 1959) 4472. [ 361 P.N. Noble and G.C. Pimentel, J. Chem. Phys. 49 (1968) 3165. [37] V.E. Bondybey and J.W. Nibler, J. Chem. Phys. 58 ( 1973) 2125.
I55 ( 1991) 267-274
Pulsed discharge source of supersonically cooled transient species R.
Schlachta, G. Lask
Institut ftir Physikalische und Theoretische Chemie der TU Miinchen. W-8046 Garching, Germany.
S.H. Tsay Department of Chemistry, The Ohio Bate University, Columbus, OH, 43210, USA
and V.E. Bondybey Institut fir Physikalische und Theoretische Chemie der TV Miinchen, W-8046 Garching, Germany and Department of Chemistry, The Ohio State University, Columbus, Ohio, 43210, USA Received 9 March I99 1
A source combining dc discharge with pulsed supersonic expansion is shown to be a versatile tool for spectroscopic studies of a variety of transient species. High concentrations of cold small radicals have been generated using suitable precursors, and detected by laser induced fluorescence. Discharging of halogenated methanes provides an efficient source of halogenated carbenes. Molecular radical cations are similarly efftciently produced, probably as a result of the Penning ionization process in collisions with metastable rare-gas atoms. Reactive sputtering of electrodes is shown to result in the production of metal halides and similar simple metal compounds. An application of the discharge source as a useful tool for matrix isolation studies is proposed and discussed.
1. Introduction Molecular ions [ l-31, free radicals [ 4-71, and other reactive, transient species [ 8,9] are important intermediates in many chemical reactions, and their properties are of considerable interest. Because of their high reactivity, however, they are usually quite short-lived, and their study represented a nontrivial experimental challenge. Traditionally, different types of electric discharges proved to be useful sources of a variety of transient species [ 7,10-201. Species formed in the discharge are usually rotationally, vibrationally and electronically quite hot, and this further complicates their spectroscopy. To improve signal to noise, and simplify the spectra, it is of course convenient to cool the species, and one way to accomplish this is matrix isolation spectroscopy. The combination of gaseous microwave or electric discharge with the matrix technique proved 0301-0104/91/%
to be a very successful combination [ 15,19 1. A novel, elegant technique, combining a corona discharge with supersonic jets was recently developed by Engelking [ 201. In his technique, a Pt electrode held at a potential of some 15 kV is located behind the orifice of a continuous Pyrex nozzle. A discharge in the expanding gas results in the formation and production of the reactive intermediates, which are cooled by adiabatic expansion. In some applications, pulsed experiments have considerable advantages over continuous systems 1211. Sustaining a pulsed supersonic expansion requires smaller and less expensive pumping systems, moreover pulsed laser systems are cheaper, more flexible, and more easily tunable over wide spectral ranges. Furthermore, one can obtain additional information about lifetimes, time evolution and dynamics, not readily obtainable in a cw laser experiment.
03.50 0 1991 - Elsevier Science Publishers B.V. (North-Holland)
268
R. Schlachta et al. / Pulsed dischargesource of transient species
In our laboratory we have been interested for some years in developing a pulsed nozzle discharge source, and the purpose of the present manuscript is to describe some of our efforts in this direction. We have designed an extremely simple source combining electric discharge with a pulsed supersonic expansion. No sophisticated power supplies or control1 circuits are required, since the opening and closing of the valve itself serves to turn on and off the discharge. Early in the development of this source several years ago our intent was to apply it to gas phase studies of Ar-OH, Ar-CN and similar van der Waals complexes, which we had previously observed in raregas matrices. These species were, in the meantime, observed and studied in several other laboratories. The free radical and ion source has, however, many other potential applications, both for the gas phase work and for low-temperature matrix studies. Here we describe our pulsed source, discuss some of its advantages, and describe several initial applications to studies of various types of supersonically cooled reaction intermediates.
R3
Fig. 1. A simplified schematic of the dc discharge control circuit. The H.V. power supply charges the capacitor C. The currents in the charging and discharging parts of the circuit are controlled by the current limiting resistors RI and R2. The opening of the valve V introduces the gaseous sample into the channel in the teflon fixture T mounted on the valve, and results in a current flowing between the metal electrodes. The operation of the valve can be monitored and the current measured by displaying the voltage drop over a small resistor R3 on the oscilloscope screen.
2. Experimental details A mixture of a suitable precursor in 3-4 bar argon expands through a pulsed valve and expands through a teflon discharge fixture mounted on the bottom of the valve. In most experiments a modified electromagnetic fuel-injection valve (Bosch) is used, although several experiments were performed with a Lasertechnics piezoelectric valve. In the presence of gas in the flow channel an electrical discharge takes place between two metal electrodes located in the low channel, as shown in fig. 1. The electrodes were made from aluminium or copper in the present experiments, but other metals could of course also be used. A voltage of typically 1000 V dc is applied at all times. A simple current circuit consisting of capacitors and current limiting resistors is used to stabilize the discharge. The resistors were chosen so as to make the time constant t = RC of the loading circuit small compared to the time between two consecutive gas pulses. During the experiment the time dependence and stability of the discharge were monitored by an oscilloscope as shown in fig. 2. After opening the electromagnetic valve, the current rises abruptly (fig.
Fig. 2. Typical current patterns observed in the apparatus in fig. 1. Using a Lasertechnics piezoelectric valve (a) or an electromagnetically driven fuel injection valve (c ) . The more complicated pattern in fig. la is due to bouncing of the valve. Fig. 2b shows the optical emission intensity from the discharge flame for the pulse of fig. 2a monitored by a photomultiplier. See text.
2~). It then decays in an exponential fashion for the duration of the gas pulse, and falls off again sharply after the valve closes. The time profile of the pulse thus provides a useful diagnostic of the operation of the valve and shows that the pulses are usually between 0.8-3 ms long, depending on the shape and duration of the driving electric pulse, The working frequency, limited by our excimer laser, is typically 5 Hz. Bouncing of the valve was a difficulty which we have experienced mainly with the piezoelectric Lasertechnics device. The problem could be reduced by adjusting the operating conditions, but we were unable to eliminate it completely. This can be seen in the fig. 2a, where the current profile suggests that for each trigger pulse the valve opens and closes twice. The reactive species expand further on through a small orifice from the discharge fixture into a vacuum chamber evacuated by a 500 a/s roots blower. The discharge naturally produces ions, electrons, radicals, and electronically excited species, and results in a visible “flame” issuing from the orifice. Further information about the operation of the discharge can be obtained by monitoring this visible emission by a photomultiplier. This is shown for the piezoelectric valve pulse of fig. 2a in the panel 2b. The time resolved profile of the emission resembles that of the discharge current, but is slightly shifted in time by several microseconds needed for the discharge products to move from the discharge through the orifice into the region monitored by the photomultiplier. It can also be noted that the emission intensity decays somewhat more rapidly than the discharge current. This may be due to the depletion of the parent species. The species present in the “flame” are probed by the pulses of an excimer (EMG 500, Lambda Physik) pumped dye laser ( 1.3 mJ per pulse, 14 ns pulse duration, FL 2000, Lambda Physik ). In some experiments, a nitrogen laser pumped dye laser was used (Molectron ) . The laser-induced fluorescence is collected by a quartz lens system and detected by a photomultiplier tube (R928, Hamamatsu) and digitized in a waveform recorder (2262, LeCroy ). The signal is then averaged and processed by a HP QS20 computer controlling the experiment. Laser excitation spectra are recorded by monitoring the undispemed fluorescence intensity as a function of the laser wave-
h
Photodiode NlM-Trigger Wavefom-Rec.
t+ Fig. 3. A timingdiagramof the experiment.The openingof the electrom~~eticvalveintroducesgasinto the discharge channel and fires the discharge. The dye laser is fired with a suitable delay after the valve is opened, and the laser induced fluorescence is digitized in a waveform recorder. The laser light is also detected by a diode, used in conjunction with a NIM delay unit to stop the digitization process, and initiate the data transfer into a computer.
length. A schematic diagram of the timing of the valve pulse, the discharge pulse, dye-laser pulse, the laserinduced fluorescence, and the detector signal is shown in fig. 3. Absolute wavenumber calibration of the dye-laser scanning system was accomplished by recording independently the spectra of the CN free radical produced from acetonitrile, and comparing the observed lines with well-known literature data. 3. Results and discussion 3. I. Production ofsimple free radicals A variety of diatomic free radicals were produced with high efficiency. For instance, intense OH, NH, CN and C2 spectra were produced by employing H20, NH, CH3CN and C2H2 as precursors. In general, the spectra exhibited rotational temperatures of 5-50 K, depending on the expansion parameters, but in many cases vibrational temperatures of several thousand Kelvin. In order to characterize the operation of the valve better, we have performed a series of experiments with aniline in the discharge, in order to be able to monitor not only the products, but also the precursor. Fig. 4a shows a typical time profile of the parent aniline fluorescence in an experiment using the pie-
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et al. / Pulsed discharge source qftransient
species
i 0
I’
LOO
800
1200
0 DELAY
LOO
800
I200
1600
lpsi
Fig. 4. Typical patterns observed by scanning the delay of the dye laser with respect to the valve opening and monitoring the fluorescence intensity. The double appearance of the signals is due to bouncing of the valve. (a) The parent aniline fluorescence; (b) the NH fragment; (c ) CN radical; (d ) C, radical.
zoelectric valve, produced by scanning the delay between the opening of the valve and the probe laser pulse. The bouncing of the Lasertechnics valve is again shown by the double appearance of the time resolved profile. When the discharge is turned on, the aniline fluorescence intensity decreases to less than one half of its initial value as a result of its decomposition, and at the same time the spectra of several products can be observed. Fig. 4b shows the time profile for NH. This is undoubtedly a primary reaction product, probably resulting from a reaction of the parent with rare-gas metastable atoms or ions. It exhibits a time profile resembling that of the parent. In addition, several other products were observed, most notably the CN and C) radicals, suggesting a fairly extensive decomposition of the aniline precursor. These products, however, exhibit time profiles considerably different from that of aniline itself, and of the NH primary product. This is probably due to the fact that they are higher order products whose production requires multiple collisions. They are therefore probably preferentially formed on the leading or trailing edge of
the pulses, where more turbulence sions can be expected.
and more colli-
3.2. Spectra of halocarbenes Fluorochlorocarbons and their dissociation products play an important part in the chemistry of the upper atmosphere and in the destruction of stratospheric ozone [ 6,22 1. In spite of the importance of the halocarbenes and other halocarbon dissociation products for atmospheric chemistry and as chemical intermediates, the spectroscopic information available about their properties and spectroscopy is still fragmentary [ 23 1. The rotationally resolved gas phase spectrum of CF2 was studied by Mathews [ 241. A number of other halocarbenes were studied by lowtemperature matrix techniques. More recently, a highresolution electronic spectrum of CCIZ was reported [ 25 1, and both CF2 and CC& were studied by microwave spectroscopy [ 26,271. Much less information is, however, available about the unsymmetrically substituted halocarbenes, CXY. In our discharge system described above, the dihalocarbenes are very efficiently produced from hal-
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R. Schlachta et al. / Pulsed discharge source of transient species
ogenated methane precursors and other halogenated organic compounds. For instance, excellent quality CCIZspectra were produced from Ccl,. An example of such a spectrum is shown in fig. 5. While the CC& properties are relatively well known [25,28], the technique is equally well applicable to other carbenes, for which spectral information is lacking. Work in this direction is currently in progress. 3.3. Production of molecular ions Above we proposed that the major mechanism of the formation of the radical fragments is the reaction of the precursor molecules with metastable rare-gas atoms. It is well documented that reactions of metastable atoms with many organic molecules result in the production of the corresponding radical cations via Penning ionization. We have tested this aspect by adding precursors whose ionization yields fluorescent molecular ions. For instance, the addition of pentafluorobenzene to the gaseous flow resulted in a visibly greenish discoloration of the discharge “flame”. Fig. 6 shows the result of scanning the dye laser and observing the resulting fluorescence. A rather strong,
I 24500
‘.
I
24600
I
24700 WAVENUMBER
24800
.
I
.
24900
[cm-‘]
Fig. 6. A short section of the fluorescence excitation spectrum of the pentatluorobenxene radical cation CsF5H+ about 1500-2000 cm-’ above the origin. As discussed in the text, the radicals are probably produced by reactions of the metastable rare-gas atoms with the parent compounds.
structured spectrum is observed, whose comparison with the published data leaves little doubt that it is due to the pentafluorobenzene radical cation [293 11. It should be noted that the region scanned in the present work corresponds to energies some 15002000 cm-’ above the origin band, where the FranckCondon factors are very weak and where the spectrum is rather congested. It is clear that excellent spectra could be produced in the region near the electronic origin, where the Franck-Condon factors are favorable. There is no spectral congestion and the bands are intense and well-resolved. 3.4. Reactive sputtering of metal atoms and spectra of simple metal compounds
m
19000
19400
19800
WAVENUMBER
20200 [cm-‘]
Fig. 5. An example of a fluorescence excitation spectrum of CClr produced by discharging about 0.1% Ccl, in argon. The spectrum consists of a series of “Fermi polyads” due to interaction between the progression of the symmetric Y, stretch with overtones of the bending frequency vr as previously reported on the basis of rare-gas matrix studies in ref. [ 281 and recently confirmed in the beautiful gas phase experiments by Clouthier and co-workers (see ref. [25] ).
The impact of ionized atoms or other reactive intermediates on the surface of the electrode may dislodge the metal atoms. This may, on the one hand, be used to investigate the reactive sputtering process as a function of the discharge parameters, such as pressure, voltage, current, nature of the carrier gas, or nature of the reactive species. On the other hand, it may serve as a valuable spectroscopic tool for studying e.g. metal oxides, sulfides, halides and other high-temperature molecules. As an example, fig. 7 exhibits the gas phase spec-
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et al. / Pulsed discharge source qf transient
specie3
previously. We find lifetimes of 320 rt 30 ns for the B+-X and 25Orfi30 ns for the C-X transition (see table 1). The techniques reported here should be quite generally applicable, and should permit a wide variety of rotationally cold halides, nitrides, oxides, carbides and similar species to be conveniently studied.
3 m =: H
3.5. Pulsed di~~char~e as a source o~tra~sie~t species of matrix isolation studies
z
W iZ f-4
L 22300
22700
23500
23100
WAVENUMBER
23900
[cm“l
Fig. 7. Spectrum showing the B ‘n-+X ‘C+ and C ‘Z+-+X ‘I+ transitions of CuBr generated by entraining about 0.1% of CFBr3 in argon flowing through the discharge and by reactive sputtering of the electrodes by the fragments produced in the discharge. The individual bands are labeied by a letter denoting the upper electronic state, and by a subscript and a superscript giving the vibrational quantum numbers U”and IJ’. Table I Molecular constants of CuBr State
r, (cm-‘)
& (cm-‘)
WJ, (cm-‘)
X B c
0 23045 23461
313.99 283.64 293.84
1.16
1.46 1.15
T (ns) 320+30 205230
trum of CuBr, obtained by entraining CF2Br2 or CFBr, in the carrier gas flowing through the discharge. Comparison with literature data [ 321 shows unequivocally that they are due to the B+-X and CtX transitions of diatomic CuBr. The obtained spectroscopic constants are given in table 1. It can be noted that similar to the other species studied, the CuBr molecules are vibrationally very hot, and levels up to V”= 5 are detected. The strong CuBr spectrum, in fact, interfered with our efforts to record and analyze the spectrum of the brom~~uor~r~ne, and for this reason we had to switch to aluminum electrodes. The spectroscopic constants of the CuBr molecule are, of course, relatively well known [ 33 3. However, as a by-product, time resolved fluorescence also provides information about the lifetimes of the observed species, and the lifetimes of CuBr were not measured
The source described above should prove to be a very convenient and efficient source of molecular ions, free radicals and other transient species for matrix isolation studies, too, and this was an additional motivation for its development. Discharges, in particular those using microwave cavities, have been frequently used in matrix studies [ 34-371. In a source of this nature, the mixture of the matrix gas with a suitable precursor flows through a 10 mm diameter quartz tube located in the microwave discharge cavity, at a pressure in the order of 0.1 Torr, and is eventually expanded through a 0.5-l mm diameter nozzle into the vacuum enclosure and deposited on the cold substrate. Under these circumstances one can easily calculate that the average precursor will remain in the discharge region for some 200 ms. If the decomposition occurs in the middle of this time period, on average, then the products will undergo some 104-lo5 collisions with other molecules and 500 wall collisions prior to being expanded through the pinhole and deposited on the cryogenic substrate. Therefore, there are many possibilities of undesired secondary recombinations or neutralization reactions. In order to circumvent this problem, alternative geometries were designed where, for instance, the pure matrix gas is discharged and mixed only shortly before the deposition and secondary reactions can be suppressed by such geometries, the extent of mixing and the conversion efficiencies can be relatively small. In the pulsed discharge source described here, on the other hand, the parent molecules are in intimate contact with the carrier gas during the discharge, but are in the nearly cotlision free jet environment a few microseconds later. After less than 1 ms, they are embedded in the solid matrix. Consequently, the conversion efficiencies can be high, but the opportunities for secondary reactions are minimized.
R. Schlachta
et al. / Pulsed discharge source of transient
In our gas phase studies with the discharge source, the typical current was 5 mA with a pulse duration of 1 ms. If one assumes that electrons are the primary current carriers, then one can calculate that approximately 5 x 1O- ’ ’ mol of electrons are needed. In such a pulse, about 2.5 umol of the gaseous mixture are deposited. Assuming a typical matrix dilution of 1: 1000, this corresponds to 2.5 nmol or 2.5 X lop9 mol of the precursor compound. If only one molecule of the precursor had to be broken up to produce each charge carrier, one could deduce that about 2% of the precursor molecules would be broken up. Actually, breaking a chemical bond typically requires much less energy (300-500 kJ/mol) than ionization ( z 1000 kJ/mol ) , and undoubtedly many more molecules are fragmented than are ionized. In the experiment using an aniline precursor, whose concentration can be conveniently monitored by laser induced fluorescence, we found that when the discharge was turned on, the parent signal dropped to about 30% of the original value, this means that about 702 of the precursor molecules were broken up. It should be noted that in the gas phase fluorescence experiments the current was held at about 4-5 mA. While at low currents the discharge was well localized between the electrodes, inside the discharge fixture and only relatively little light from the discharge could reach the detector, when it was increased beyond this value, the character of the discharge suddenly changed. The glow discharge extended far into the vacuum chamber and produced a bright luminescent “flame”. This emission, which was of course bothersome in the gas phase LIF studies, would not be a problem in matrix work. Consequently, if a more complete decomposition of the sample were desired, the currents could easily be increased by at least one additional order of magnitude. Similar to the pulsed laser vaporization source we recently described, the pulsed discharge should also permit a much quicker generation of transient species in sufficient concentrations for spectroscopic characterization. Operating the source at only 2 pps, as in the above experiments, will result in deposits in the order of 2-5 mmol in about 10 min, whereas in conventional continuous deposition experiments several hours are usually required. An additional advantage of the pulsed operation is that unlike continuous deposition it provides clear, unscattering de-
species
273
posits of excellent optical quality even with heavier matrix gases such as argon or krypton. The pulsed discharge unit of the type described here should therefore provide an extremely useful source of transient species for matrix isolation studies, providing the advantages of a more complete precursor conversion and much less opportunity for secondary reactions. We are currently in the process of building a matrix setup which should soon allow us to test the source in matrix experiments.
4. Summary A very simple dc discharge source for spectroscopic studies of supersonically cooled ions, radicals and other transient species has been constructed, and its operation and applications have been described. We have shown that, using suitable precursor molecules, extremely high concentrations of cold reactive intermediates can be produced and detected by time resolved pulsed laser induced fluorescence. Various halocarbenes can be produced by discharging halogenated methanes. The Penning ionization process by the rare-gas metastables produces molecular radical cations similarly efficiently. Simple metal halides can be produced in high yields by reactive sputtering of the metal electrodes, and by using suitable reactants the technique can be applied to produce other simple metal compounds. A potential application of the discharge source as an efficient tool for matrix isolation studies is proposed and discussed.
Acknowledgements This work was supported in part by the National Science Foundation under grant no. CHE8803169 and by The Ohio State University Research Grant Program. Assistance by the Fonds der Chemischen Industrie is also gratefully acknowledged.
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