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Infrared spectroscopy of the products of a corona-excited supersonic expansion
Volume 202, number 3.4
CHEMICAL PHYSICS LETTERS
22 January I993
Infrared spectroscopy of the products of a corona-excited supersonic expansion Kenneth R. Comer and Stephen C. Foster Departmpnl ofChemistry, Florida State University, Tallahassee, FL 32306-3006, USA
Received 7 August 1992; in final form 27 October 1992
A novel corona-excited supersonic jet slit discharge is described, and its use as a source of rotationally cold free radicals for infrared direct absorption spectroscopy is discussed. The system was tested by detecting the rovibrational spectrum of CS and by observing the v2band of NH2 3 ‘B,. Two different phase sensitive detection schemes were used. Although conventional frequency modulation of the infrared laser was successfully employed, a new population modulation scheme was more effective. Under the conditions described, rotational temperatures of the detected fragments were ~40 K.
1. Introduction The supersonic free jet expansion has become an important spectroscopic tool, allowing the study of rotationally resolved spectra of large molecules at low temperature. The spectral simplification and stabilizing environment associated with the cold rare-gas expansion usually cannot be achieved in any other way. Free jets are routinely used to study stable molecules and clusters formed from stable molecules (for example see ref. [ 1 ] ). Methods have also been developed to study the spectra of unstable species in free jets (for example see ref. [ 21) [ 3 1. The majority of these jet-based spectroscopic studies have been performed in the ultraviolet and visible regions because of the sensitivity of fluorescence-based detection schemes. Infrared studies of molecular fragments in supersonic free jets are much less common [ 4-71, although there is a very active research effort into the spectra of stable molecules and molecular clusters in this wavelength region. The majority of the UV/visible laser studies are based upon laser photofragmentation and/or photoionization of a stable precursor and are necessarily pulsed production methods. The pulsed production, and the rapid stream velocities of these expansions (2x lO’ms-’ for a 300K helium reservoir) severely limit the time that a laser can observe the species of interest. If the laser probes a 1 mm length of 216
the expansion, fragments can only be detected for 0.5 ps after the generation pulse ends. Such low duty cycle pulsed methods cannot fully exploit the capabilities of cw high-resolution tunable infrared lasers. Continuous sources of molecular fragments are clearly desirable. Two jet-based continuous fragment production schemes have enjoyed some success over the past few years: The corona-excited free jet expansion as pioneered by Engelking [ 8 ] and the thermal dissociation nozzle [ 9-121. We have chosen to develop a corona source for our studies. It is effecrive in generating numerous molecular fragments [3] and it lends itself to a high-frequency population modulation scheme to enhance sensitivity. This Letter details the new jet design and describes our initial success in the detection of molecular fragments.
2. Corona slit discharge A conventional corona-excited discharge [ 81 consists of a thin metal rod, terminated with a sharp point, mounted inside an insulating and inert tube. A small pinhole or slit is cut into the end of the tube, forming the supersonic nozzle. The metal tip is placed very close to the exit hole and a high positive voltage is applied to the rod. If the chamber or pump is grounded while a dilute mixture of a precursor mol-
0009-2614/93/$06.00 0 1993 Elsevier Science Publishers B.V. All rights reserved.
Volume 202, number 3,4
CHEMICAL PHYSICS LETTERS
ecule in a rare gas is expanded through the orifice, an electrical discharge occurs and molecular fragments and electronically excited precursor molecules are produced. The nozzle body is usually Pyrex or quartz tubing. Pin holes are produced by drawing out the tubing until it closes and then grinding away excess glass until a small hole is revealed. Slit nozzles are more difficult to prepare, but offer some advantages. Although the column density along a path parallel to the long axis of the slit is only slightly larger ( 17O/o) than that offered by a circular nozzle of equal mass throughout [ 131, the smaller Doppler widths associated with the planar expansion increase resolution and peak intensity. Milkman et al. [ 141 have generated slits in the thin-walled end of a closed tube by laser ablation, but we found it difficult to produce slits with large aspect ratios in this way. Furthermore, the discharge currents used here (G l-5 mA) rapidly enlarged the openings, which severely limited the slit lifetimes. An alternative slit design is now in use, see fig. 1. Razor blades (X-Act0 No. 18, large chisel) are mounted on the surface of a teflon block to define the sides of the slit. A gas tight seal is formed by an O-ring sandwiched between the blades and the block, with the inner diameter of the O-ring defining the length of the slit. The division of the teflon block into two sections gives flexibility in the choice of this Oring and hence flexibility in slit length. The main block assembly is attached to a thick-walled Pyrex tube with an O-ring sealed compression fitting, Although the slit is no longer insulating in this design, it is isolated. The blades are charged by the plasma, but these charged plates presumably act as a focusing
G Tf
22 January 1993
lens for the electron beam. This new design has proven to be very robust and it is simple to generate 30 pm wide slits of almost arbitrary length in this way. The expansion gas is supplied from an ambient temperature 40 psig rare-gas reservoir seeded with a small quantity of a stable precursor. The vacuum chamber is constructed from schedule-40 polyvinyl chloride tubing and is evacuated with an Edwards EH500/E2M40 Roots-blown pumping stack. The nominal pumping speed for this system is 117 IIs-’ but is reduced by a protective screen placed above the blower. The operating chamber pressure is typically < 1 Torr. A high voltage power supply is connected to the central electrode via a 3 ML! ballast resistor. Unlike conventional corona systems, the discharge is established at low voltage (less than 1000 V) and will operate over a large range of currents. The image charge of the electrode in the slit blades and the proximity of the blades to the lowpressure chamber may account for this behavior. With a +9 kV applied potential (3 mA current) the central electrode was measured to be z 350 V relative to ground and the razors charged to R 200 V. An infrared diode laser (Laser Analytics) provides the tunable radiation used to detect molecular fragments. The laser beam is directed into the chamber and makes two passes through the jet ( = 1 mm downstream from the slit). The output beam is sent through a 1 m mode-separation monochromator, which also serves as a narrow bandpass filter of any emission from the discharge, and is then split into two parts. One portion (10% of the beam) is directed through an ttalon and onto a mercury cadmium telluride detector (MCT). The remaining 90% passes through a reference-gas cell and then onto a second MCT. The Ctalon is a home-built 30 cm airspaced internally coupled device, similar in design to the system described by Reich et al. [ 151. Design changes include plane-parallel, rather than confocal, mirrors and a heated NaCl beamsplitter.
3. Detection schemes Fig. 1. Cross section and front view of the corona slit jet. All parts are teflon, with the exception of G (thick wall Pyrex tubing, 6 mm outer diameter), T (tantalum wire, 0.38 mm diameter), X (Xacto blades) and nylon screws.
Conventional source frequency modulation of the laser coupled with 2-f phase sensitive amplification of the detector output was used to detect species produced in the corona jet. An example of a typical ab217
CHEMICAL PHYSICS LETTERS
Volume202,number3,4
sorption line ( 1i ,-Ooo,F, spin component) of the vz band of the NH2 radical is shown in fig. 2. The signal-to-noise is not very high, but the “noise” was reproducible and was largely due to small frequencydependent interference effects between the various optical elements and surfaces in the system (Ctalons). This is a common problem in high-sensitivity infrared absorption experiments and these effects are typically suppressed by using a molecule specific modulation scheme with subsequent phase sensitive amplification. Concentration [ 161, velocity [ 171, and Zeeman and Stark modulation (for example see ref. [ 181) are examples of this procedure. In concentration modulation the fragment concentration is modulated when a discharge is switched on and off at high frequency, and it is in principle the best molecule specific scheme. It works equally well for ionic and neutral fragments, does not require an unpaired electron and the modulation efficiency is rotational-state independent. In a conventional discharge, however, the maximum modulation frequency is inversely proportional to the lifetime of the species and relatively long-lived species can only be modulated at low frequency with less effective noise rejection. This is not the case in a jet discharge. Fragments are produced during the on half of the cycle, as before, but the depletion rate is now much higher once the discharge turns off. In an ex-
'W
"2
111%
‘ur-““I
I
1531.16
1531.20 Cm-'
Fig. 2. The NH2 2 2B, v2, 1,,-O,, F, line. 10kHz fm, 2-f deteo tion. The scan rate was 0.001 cm-’ s-’ and the time constant was 300 ms. The line was observed in an Ar/NHx (trace quantity) discharge (3 mA current ) through a 60 pm x 4 mm slit. The stagnation pressure was 40 psig and the chamber pressure was 0.9 Torr.
218
22 January1993
pansion, molecules are carried from the viewing region at high velocity (in helium x 2 x 1O3m s- ’) , and when the discharge is extinguished the fragments rapidly disappear. Thus the maximum modulation rate is nut set by the fragment lifetime, but by the rate at which the fragments are swept from the region probed by the laser. The helium stream velocity and a I mm interaction length imply an upper limit of = 2 MHz. In most cases the maximum rate will be determined by the detector frequency response, but rates above 100 kHz should be possible with fast detectors and will strongly suppress laser and detector noise. In practice it is difficult to switch high voltages at high frequencies. The large ballast resistance and capacitance of the metal electrode preclude rates above z 1 kHz, which is too low to be of interest. However, the present jet design provides an attractive alternative way to modulate the discharge. When the metal slits of the nozzle were grounded the discharge was not extinguished, but the visible emission was attenuated and the plasma “flame” spatial outline was distorted. A 50k reduction in the NH2 absorption signal was detected when the slits were grounded. The a200V isolated slit voltage can easily be held off with simple transistors. Thus fragment concentrations were modulated by connecting the collector of a Philips ECG-287 npn silicon power transistor to the slit blades, applying a 4 V square wave to the base (with a 1 kR pull-down resistor), and connecting the emitter to ground. The discharge current, and hence fragment concentration, was modulated at the frequency of the low voltage square wave applied to the transistor base.
4. Results Large improvements in signal to noise were achieved with the new modulation scheme described above. Fig. 3 shows the NH2 line presented earlier (fig. 2), but recorded with 25 kHz discharge modulation and l-f detection. The signal to noise enhancement is large, and strong suppression of the stable NH3 fundamental band absorption is evident. Discharge modulation was also demonstrated at 50 kHz and this upper limit was restricted by the frequency response of the detector.
Volume 202, number 3,4
CHEMICAL PHYSICS LETTERS
NH, ~4
opP(6,) I
I
I
1531.10
1531.15
1531.20
cm
-1
.I ‘he NH2 3 *B, vl, 1, ,-Om F, line recorded with 2.5kHz discharge modulation and 1-f phase-sensitive detection. All other conditions are described in tip. 2.
Several mechanisms can lead to the amplification of the NH3 line. The concentration of NH3 is modulated as NH3 is destroyed in the discharge, but these signals should occur 180” out of phase with the NHp production signals. The NH3 may also be temperature modulated: with the discharge off, the rotational temperature may be lower than with the discharge on. Ammonia could be heated by collision with the initially hot NH2 fragments, alternatively some NH3 molecules could be directly heated by the electron beam. Neither mechanism seems important here, and the residual NH3 signal was probably caused by either the incomplete rejection of the very strong NH3 absorption by the phase-sensitive amplifier, or because the laser controller was not completely shielded from broadcast rf noise. In earlier fm-based studies with this corona slit system, the fundamental band of CS X ‘Z was detected in a discharge of a 5% mixture of CS2 in helium expanded through a 5 mm x 75 pm slit. With a 40 psig reservoir pressure and 3.5 mA discharge current the rotational temperature for this fragment was found to be 40 K. This is not atypical for a diatomic fragment [ 19-22 1. Hot vibrational distributions are prominent in emission studies of corona jets [ 19221, but no hot transitions were observed here and hence no estimate of the vibrational temperature of the discharge can be made. The line shapes shown in figs. 2 and 3 have sharp
22 January 1993
maxima, indicating the absence of large-scale clustering under these conditions. Circular nozzles have been observed to produce flat-topped lines, characteristic of signal depletion as the coldest on-axis molecules preferentially form clusters [ 13,231. If desired, it may be possible to induce significant clustering by operating at higher pressures and through the use of thicker slit blades. The thicker blades will form a longer expansion channel thereby enhancing clustering. The full-width half-maximum line width of the discharge modulated NH2 lines is measured to be O.O046cm-i, corresponding to a translational temperature of 155 K, although the observed line is probably broadened by unresolved hyperfine structure due to the 14Nnuclear spin (I,= 1) as well as the resultant proton spin (Zn= 1) for the ortho levels involved.
5. Conclusions
The present work demonstrates the utility of a newly designed corona excited slit discharge as a source of molecular radicals for infrared direct absorption spectroscopy. The new design offers the significant advantage of a very high-frequency concentration modulation scheme for signal enhancement and precursor suppression. The technique is simple to apply and suggests several exciting possibilities. The cooling and linewidth reduction should allow the study of much larger species. The techniques described are general and it should be possible to also detect molecular ions and molecules in metastable states with long radiative lifetimes.
Acknowledgement
Acknowledgement is made to the Donors of The Petroleum Research Fund, administered by the American Chemical Society, for support of this research. 219
Volume 202, number 3,4
CHEMICAL PHYSICS LETTERS
References
M. Ito, T. Ebata and N. Mikama, Ann. Rev. Phys. Chem. 39 (1988) 123; D.J. Nesbitt, Chem. Rev. 88 (1988) 843; F.G. Celli and KC. Janda, Chem. Rev. 86 (1986) 507; A.C. Legon and D.J. Millen, Chem. Rev. 86 (1986) 635. M. Heaven, Ann. Rev. Phys. Chem. 43, in press; S.C. Foster, R.A. Kennedy and T.A. Miller, The laser spectroscopy of chemical intermediates in supersonic free jet expansions, in: Proceedings of the NATO-ASI, Frontiers of laser spectroscopy of gases, eds. P. Alves et al. (Kluwer, Dordrecht, 1988) p, 421. [3] P.C. Engelking, Chem. Rev. 91 (1991) 339. [4] R.F. Curl, K.K. Murray, M. Petri, M.L. Richnow and F.K. Tittle, Chem. Phys. Letters 161 (1989) 98. [5 ] R.C. Cohen, K.L. Busarow, CA. Schuttenmaer, Y.T. Lee and R.J. Saykally, Chem. Phys. Letters 164 (1989) 321. [6]J.V. Coe, J.C. Owrutsky, E.R. Keim, N.V. Agman, D.C. Hovde and R.J. Saykally, J. Chem. Phys. 90 (1989) 3893. [7] J.R. Heath and R.J. Saykally, J. Chem. Phys. 93 (1990) 8392; 94 (1991) 1724,327l. [8] P.C. Engelking, Rev. Sci. Instr. 57 (1986) 2274. [9] B. Brutschy and H. Haberland, J. Phys. E 13 (1980) 150. [IO] P. Chen, S.D. Coulson, W.A. Chupka and J.A. Berson, J. Phys. Chem. 90 ( 1986) 23 19.
220
22 January 1993
[ I1 ] J.R. Dunlop, J. Karolczak and D.J. Clouthier, Chem. Phys. Letters 151 (1988) 362. [ 121J.-I. Choe, S.R. Tanner and M.D. Harmony, J. Mol. Spectry. 138 (1989) 319. [ 131K. Veeken and J. Reuss, Appl. Phys. B 38 ( 1985) 117. [ 1411.W. Milkman, J.C. Choi, J.L. Hardwick and J.T. Mosely, Rev. Sci. Instr. 59 ( 1988) 508. [ 151M. Reich, R. Scheider, H.J. Clar and G. Winnewisser, Appl. Opt. 25 (1986) 130. [ 161C. Yamada and E. Hirota, J. Chem. Phys. 78 ( 1983) 669. [ 17] C.S. Gudeman, M.H. Begemann, I. Pfaff and R.J. Saykally, Phys. Rev. Letters 50 (1983) 727. [ 1S] E. Hirota, in; Chemical and biochemical applications of lasers, ed. C.B. Moore (Academic Press, New York, 1980) ch. 5. [ 19] P.G. Carrick and P.C. Engelking,J. Chem. Phys. 8 1 ( 1984) 1661. 1201S.D. Brossard, P.G. Carrick, EL. Chappell, S.C. Hulegard and PC. Engelking, J. Chem. Phys. 84 ( 1986) 2459. [2 1 ] E.L. Chappell and P.C. Engelking,J. Chem. Phys. 89 (1988) 6007. [22] A.R. Hoy, K.J. Jordan and R.H. Lipson, J. Phys. Chem. 95 (1991) 611. [23] M. Mizugai, H. Kuze, H. Jones and M. Takami, Appl. Phys. B32 (1983)43.
CHEMICAL PHYSICS LETTERS
22 January I993
Infrared spectroscopy of the products of a corona-excited supersonic expansion Kenneth R. Comer and Stephen C. Foster Departmpnl ofChemistry, Florida State University, Tallahassee, FL 32306-3006, USA
Received 7 August 1992; in final form 27 October 1992
A novel corona-excited supersonic jet slit discharge is described, and its use as a source of rotationally cold free radicals for infrared direct absorption spectroscopy is discussed. The system was tested by detecting the rovibrational spectrum of CS and by observing the v2band of NH2 3 ‘B,. Two different phase sensitive detection schemes were used. Although conventional frequency modulation of the infrared laser was successfully employed, a new population modulation scheme was more effective. Under the conditions described, rotational temperatures of the detected fragments were ~40 K.
1. Introduction The supersonic free jet expansion has become an important spectroscopic tool, allowing the study of rotationally resolved spectra of large molecules at low temperature. The spectral simplification and stabilizing environment associated with the cold rare-gas expansion usually cannot be achieved in any other way. Free jets are routinely used to study stable molecules and clusters formed from stable molecules (for example see ref. [ 1 ] ). Methods have also been developed to study the spectra of unstable species in free jets (for example see ref. [ 21) [ 3 1. The majority of these jet-based spectroscopic studies have been performed in the ultraviolet and visible regions because of the sensitivity of fluorescence-based detection schemes. Infrared studies of molecular fragments in supersonic free jets are much less common [ 4-71, although there is a very active research effort into the spectra of stable molecules and molecular clusters in this wavelength region. The majority of the UV/visible laser studies are based upon laser photofragmentation and/or photoionization of a stable precursor and are necessarily pulsed production methods. The pulsed production, and the rapid stream velocities of these expansions (2x lO’ms-’ for a 300K helium reservoir) severely limit the time that a laser can observe the species of interest. If the laser probes a 1 mm length of 216
the expansion, fragments can only be detected for 0.5 ps after the generation pulse ends. Such low duty cycle pulsed methods cannot fully exploit the capabilities of cw high-resolution tunable infrared lasers. Continuous sources of molecular fragments are clearly desirable. Two jet-based continuous fragment production schemes have enjoyed some success over the past few years: The corona-excited free jet expansion as pioneered by Engelking [ 8 ] and the thermal dissociation nozzle [ 9-121. We have chosen to develop a corona source for our studies. It is effecrive in generating numerous molecular fragments [3] and it lends itself to a high-frequency population modulation scheme to enhance sensitivity. This Letter details the new jet design and describes our initial success in the detection of molecular fragments.
2. Corona slit discharge A conventional corona-excited discharge [ 81 consists of a thin metal rod, terminated with a sharp point, mounted inside an insulating and inert tube. A small pinhole or slit is cut into the end of the tube, forming the supersonic nozzle. The metal tip is placed very close to the exit hole and a high positive voltage is applied to the rod. If the chamber or pump is grounded while a dilute mixture of a precursor mol-
0009-2614/93/$06.00 0 1993 Elsevier Science Publishers B.V. All rights reserved.
Volume 202, number 3,4
CHEMICAL PHYSICS LETTERS
ecule in a rare gas is expanded through the orifice, an electrical discharge occurs and molecular fragments and electronically excited precursor molecules are produced. The nozzle body is usually Pyrex or quartz tubing. Pin holes are produced by drawing out the tubing until it closes and then grinding away excess glass until a small hole is revealed. Slit nozzles are more difficult to prepare, but offer some advantages. Although the column density along a path parallel to the long axis of the slit is only slightly larger ( 17O/o) than that offered by a circular nozzle of equal mass throughout [ 131, the smaller Doppler widths associated with the planar expansion increase resolution and peak intensity. Milkman et al. [ 141 have generated slits in the thin-walled end of a closed tube by laser ablation, but we found it difficult to produce slits with large aspect ratios in this way. Furthermore, the discharge currents used here (G l-5 mA) rapidly enlarged the openings, which severely limited the slit lifetimes. An alternative slit design is now in use, see fig. 1. Razor blades (X-Act0 No. 18, large chisel) are mounted on the surface of a teflon block to define the sides of the slit. A gas tight seal is formed by an O-ring sandwiched between the blades and the block, with the inner diameter of the O-ring defining the length of the slit. The division of the teflon block into two sections gives flexibility in the choice of this Oring and hence flexibility in slit length. The main block assembly is attached to a thick-walled Pyrex tube with an O-ring sealed compression fitting, Although the slit is no longer insulating in this design, it is isolated. The blades are charged by the plasma, but these charged plates presumably act as a focusing
G Tf
22 January 1993
lens for the electron beam. This new design has proven to be very robust and it is simple to generate 30 pm wide slits of almost arbitrary length in this way. The expansion gas is supplied from an ambient temperature 40 psig rare-gas reservoir seeded with a small quantity of a stable precursor. The vacuum chamber is constructed from schedule-40 polyvinyl chloride tubing and is evacuated with an Edwards EH500/E2M40 Roots-blown pumping stack. The nominal pumping speed for this system is 117 IIs-’ but is reduced by a protective screen placed above the blower. The operating chamber pressure is typically < 1 Torr. A high voltage power supply is connected to the central electrode via a 3 ML! ballast resistor. Unlike conventional corona systems, the discharge is established at low voltage (less than 1000 V) and will operate over a large range of currents. The image charge of the electrode in the slit blades and the proximity of the blades to the lowpressure chamber may account for this behavior. With a +9 kV applied potential (3 mA current) the central electrode was measured to be z 350 V relative to ground and the razors charged to R 200 V. An infrared diode laser (Laser Analytics) provides the tunable radiation used to detect molecular fragments. The laser beam is directed into the chamber and makes two passes through the jet ( = 1 mm downstream from the slit). The output beam is sent through a 1 m mode-separation monochromator, which also serves as a narrow bandpass filter of any emission from the discharge, and is then split into two parts. One portion (10% of the beam) is directed through an ttalon and onto a mercury cadmium telluride detector (MCT). The remaining 90% passes through a reference-gas cell and then onto a second MCT. The Ctalon is a home-built 30 cm airspaced internally coupled device, similar in design to the system described by Reich et al. [ 151. Design changes include plane-parallel, rather than confocal, mirrors and a heated NaCl beamsplitter.
3. Detection schemes Fig. 1. Cross section and front view of the corona slit jet. All parts are teflon, with the exception of G (thick wall Pyrex tubing, 6 mm outer diameter), T (tantalum wire, 0.38 mm diameter), X (Xacto blades) and nylon screws.
Conventional source frequency modulation of the laser coupled with 2-f phase sensitive amplification of the detector output was used to detect species produced in the corona jet. An example of a typical ab217
CHEMICAL PHYSICS LETTERS
Volume202,number3,4
sorption line ( 1i ,-Ooo,F, spin component) of the vz band of the NH2 radical is shown in fig. 2. The signal-to-noise is not very high, but the “noise” was reproducible and was largely due to small frequencydependent interference effects between the various optical elements and surfaces in the system (Ctalons). This is a common problem in high-sensitivity infrared absorption experiments and these effects are typically suppressed by using a molecule specific modulation scheme with subsequent phase sensitive amplification. Concentration [ 161, velocity [ 171, and Zeeman and Stark modulation (for example see ref. [ 181) are examples of this procedure. In concentration modulation the fragment concentration is modulated when a discharge is switched on and off at high frequency, and it is in principle the best molecule specific scheme. It works equally well for ionic and neutral fragments, does not require an unpaired electron and the modulation efficiency is rotational-state independent. In a conventional discharge, however, the maximum modulation frequency is inversely proportional to the lifetime of the species and relatively long-lived species can only be modulated at low frequency with less effective noise rejection. This is not the case in a jet discharge. Fragments are produced during the on half of the cycle, as before, but the depletion rate is now much higher once the discharge turns off. In an ex-
'W
"2
111%
‘ur-““I
I
1531.16
1531.20 Cm-'
Fig. 2. The NH2 2 2B, v2, 1,,-O,, F, line. 10kHz fm, 2-f deteo tion. The scan rate was 0.001 cm-’ s-’ and the time constant was 300 ms. The line was observed in an Ar/NHx (trace quantity) discharge (3 mA current ) through a 60 pm x 4 mm slit. The stagnation pressure was 40 psig and the chamber pressure was 0.9 Torr.
218
22 January1993
pansion, molecules are carried from the viewing region at high velocity (in helium x 2 x 1O3m s- ’) , and when the discharge is extinguished the fragments rapidly disappear. Thus the maximum modulation rate is nut set by the fragment lifetime, but by the rate at which the fragments are swept from the region probed by the laser. The helium stream velocity and a I mm interaction length imply an upper limit of = 2 MHz. In most cases the maximum rate will be determined by the detector frequency response, but rates above 100 kHz should be possible with fast detectors and will strongly suppress laser and detector noise. In practice it is difficult to switch high voltages at high frequencies. The large ballast resistance and capacitance of the metal electrode preclude rates above z 1 kHz, which is too low to be of interest. However, the present jet design provides an attractive alternative way to modulate the discharge. When the metal slits of the nozzle were grounded the discharge was not extinguished, but the visible emission was attenuated and the plasma “flame” spatial outline was distorted. A 50k reduction in the NH2 absorption signal was detected when the slits were grounded. The a200V isolated slit voltage can easily be held off with simple transistors. Thus fragment concentrations were modulated by connecting the collector of a Philips ECG-287 npn silicon power transistor to the slit blades, applying a 4 V square wave to the base (with a 1 kR pull-down resistor), and connecting the emitter to ground. The discharge current, and hence fragment concentration, was modulated at the frequency of the low voltage square wave applied to the transistor base.
4. Results Large improvements in signal to noise were achieved with the new modulation scheme described above. Fig. 3 shows the NH2 line presented earlier (fig. 2), but recorded with 25 kHz discharge modulation and l-f detection. The signal to noise enhancement is large, and strong suppression of the stable NH3 fundamental band absorption is evident. Discharge modulation was also demonstrated at 50 kHz and this upper limit was restricted by the frequency response of the detector.
Volume 202, number 3,4
CHEMICAL PHYSICS LETTERS
NH, ~4
opP(6,) I
I
I
1531.10
1531.15
1531.20
cm
-1
.I ‘he NH2 3 *B, vl, 1, ,-Om F, line recorded with 2.5kHz discharge modulation and 1-f phase-sensitive detection. All other conditions are described in tip. 2.
Several mechanisms can lead to the amplification of the NH3 line. The concentration of NH3 is modulated as NH3 is destroyed in the discharge, but these signals should occur 180” out of phase with the NHp production signals. The NH3 may also be temperature modulated: with the discharge off, the rotational temperature may be lower than with the discharge on. Ammonia could be heated by collision with the initially hot NH2 fragments, alternatively some NH3 molecules could be directly heated by the electron beam. Neither mechanism seems important here, and the residual NH3 signal was probably caused by either the incomplete rejection of the very strong NH3 absorption by the phase-sensitive amplifier, or because the laser controller was not completely shielded from broadcast rf noise. In earlier fm-based studies with this corona slit system, the fundamental band of CS X ‘Z was detected in a discharge of a 5% mixture of CS2 in helium expanded through a 5 mm x 75 pm slit. With a 40 psig reservoir pressure and 3.5 mA discharge current the rotational temperature for this fragment was found to be 40 K. This is not atypical for a diatomic fragment [ 19-22 1. Hot vibrational distributions are prominent in emission studies of corona jets [ 19221, but no hot transitions were observed here and hence no estimate of the vibrational temperature of the discharge can be made. The line shapes shown in figs. 2 and 3 have sharp
22 January 1993
maxima, indicating the absence of large-scale clustering under these conditions. Circular nozzles have been observed to produce flat-topped lines, characteristic of signal depletion as the coldest on-axis molecules preferentially form clusters [ 13,231. If desired, it may be possible to induce significant clustering by operating at higher pressures and through the use of thicker slit blades. The thicker blades will form a longer expansion channel thereby enhancing clustering. The full-width half-maximum line width of the discharge modulated NH2 lines is measured to be O.O046cm-i, corresponding to a translational temperature of 155 K, although the observed line is probably broadened by unresolved hyperfine structure due to the 14Nnuclear spin (I,= 1) as well as the resultant proton spin (Zn= 1) for the ortho levels involved.
5. Conclusions
The present work demonstrates the utility of a newly designed corona excited slit discharge as a source of molecular radicals for infrared direct absorption spectroscopy. The new design offers the significant advantage of a very high-frequency concentration modulation scheme for signal enhancement and precursor suppression. The technique is simple to apply and suggests several exciting possibilities. The cooling and linewidth reduction should allow the study of much larger species. The techniques described are general and it should be possible to also detect molecular ions and molecules in metastable states with long radiative lifetimes.
Acknowledgement
Acknowledgement is made to the Donors of The Petroleum Research Fund, administered by the American Chemical Society, for support of this research. 219
Volume 202, number 3,4
CHEMICAL PHYSICS LETTERS
References
M. Ito, T. Ebata and N. Mikama, Ann. Rev. Phys. Chem. 39 (1988) 123; D.J. Nesbitt, Chem. Rev. 88 (1988) 843; F.G. Celli and KC. Janda, Chem. Rev. 86 (1986) 507; A.C. Legon and D.J. Millen, Chem. Rev. 86 (1986) 635. M. Heaven, Ann. Rev. Phys. Chem. 43, in press; S.C. Foster, R.A. Kennedy and T.A. Miller, The laser spectroscopy of chemical intermediates in supersonic free jet expansions, in: Proceedings of the NATO-ASI, Frontiers of laser spectroscopy of gases, eds. P. Alves et al. (Kluwer, Dordrecht, 1988) p, 421. [3] P.C. Engelking, Chem. Rev. 91 (1991) 339. [4] R.F. Curl, K.K. Murray, M. Petri, M.L. Richnow and F.K. Tittle, Chem. Phys. Letters 161 (1989) 98. [5 ] R.C. Cohen, K.L. Busarow, CA. Schuttenmaer, Y.T. Lee and R.J. Saykally, Chem. Phys. Letters 164 (1989) 321. [6]J.V. Coe, J.C. Owrutsky, E.R. Keim, N.V. Agman, D.C. Hovde and R.J. Saykally, J. Chem. Phys. 90 (1989) 3893. [7] J.R. Heath and R.J. Saykally, J. Chem. Phys. 93 (1990) 8392; 94 (1991) 1724,327l. [8] P.C. Engelking, Rev. Sci. Instr. 57 (1986) 2274. [9] B. Brutschy and H. Haberland, J. Phys. E 13 (1980) 150. [IO] P. Chen, S.D. Coulson, W.A. Chupka and J.A. Berson, J. Phys. Chem. 90 ( 1986) 23 19.
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