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Tunable diode laser spectrometer for pulsed supersonic jets: application to weakly-bound complexes and clusters
Spectrochimica Acta Part A 60 (2004) 3235–3242
Tunable diode laser spectrometer for pulsed supersonic jets: application to weakly-bound complexes and clusters Matthew D. Brookes, Changhong Xia, Jian Tang, James A. Anstey, Bryan G. Fulsom, Ke-Xian Au Yong, Jenna M. King, A.R.W. McKellar∗ Steacie Institute for Molecular Sciences, National Research Council of Canada, Ottawa Ont., Canada K1A 0R6 Received 4 September 2003; accepted 12 November 2003
Abstract The design and operation of an apparatus for studying infrared spectra of weakly-bound complexes is described in detail. A pulsed supersonic jet expansion is probed using a tunable Pb-salt diode laser spectrometer operated in a rapid-scan mode. The jet may be fitted with either pinhole or slit shaped nozzles, the former giving lower effective rotational temperatures, and the latter giving sharper spectral lines. Notable features of the apparatus include use of a toroidal multi-pass mirror system to give over 100 passes of the laser through the supersonic jet, use of the normal laser controller for laser sweeping during both setup and data acquisition, and use of a simple semi-automated wavenumber calibration procedure. Performance of the apparatus is illustrated with observed spectra of the van der Waals complex He–OCS, and the seeded helium clusters HeN –OCS and HeN –CO. Published by Elsevier B.V. Keywords: Infrared spectra; Spectral lines; Laser spectrometer; Tunable diode laser; Weakly-bound molecules; van der Waals molecules; Data acquisition systems
1. Introduction During the past 20 years, high resolution infrared spectroscopy of weakly-bound van der Waals complexes has emerged as one of the most direct and precise approaches to the measurement of intermolecular forces. In our laboratory, we have studied such spectra with a variety of techniques, for example using gas samples under equilibrium (e.g. cooled long path gas cells) or non-equilibrium (e.g. supersonic jet) conditions, and probing these samples with Fourier transform spectrometers (FTIR) or with tunable laser sources. Our first supersonic jet apparatus utilized a continuous expansion through a slit jet nozzle, as probed by an infrared Pb-salt tunable diode laser (TDL) spectrometer operating in a ‘slow-scan’ (≈0.001 cm−1 s−1 ) mode. Although this gave some excellent results [1,2], it required very large vacuum pumps and consumed very large amounts of sample gas. As an alternative, we started around 1997 to develop a pulsed-jet apparatus using the same TDL spectrometer in a rapid-scan (≈1 cm−1 ms−1 ) mode. Beginning with spectra of the CO
dimer [3], this apparatus has been used to study a number of binary van der Waals complexes [4–18], and, more recently, larger clusters [19–22] containing a number of helium atoms and a single infrared chromophore molecule. The design of this pulsed-jet apparatus and its subsequent evolution is the subject of the present article, together with a sample of some recent results.
2. The original design The first experiment using a tunable infrared laser to study direct infrared absorption spectra of complexes in a pulsed supersonic jet expansion was reported by Hayman et al. [23] in 1985. Many other researchers have subsequently applied this technique, notably Nesbitt and co-workers [24]. Our apparatus was based on the rapid-scan technique introduced by De Piante et al. [25] and also used in a number of other labs [26–28]. 2.1. Vacuum system and supersonic jet
∗
Corresponding author. Tel.: +1-613-990-0736; fax: +1-613-991-2648. E-mail address: [email protected] (A.R.W. McKellar). 1386-1425/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.saa.2003.11.041
The jet chamber is a standard ISO NW250 six-way cross, mounted on a gate valve which in turn is mounted on a
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Varian VHS-10 diffusion pump. This assembly is supported on a 1 in. thick aluminum plate attached to the side of a 1.2×2.4 m optical table, so that the center line of the six-way cross is approximately 43 cm above the table surface. The diffusion pump is backed by Edwards EH500 Roots booster and E2M40 rotary pumps which are located in a separate room at a distance of approximately 4 m. During jet operation, the average gas flow can be conveniently monitored using an ordinary thermocouple vacuum gauge located in the backing line. We use a solenoid-actuated jet which is located entirely inside the vacuum chamber, supported by its 6 mm. gas supply tube. This mounting tube passes through a bored-out 6 mm. Cajon Ultra-Torr fitting in the center of the top plate of the chamber so that the jet fires downward, directly into the diffusion pump. The distance from the jet nozzle to the probe laser beam can be varied by simply raising or lowering the mounting tube. Both pinhole- and slit-shaped jet nozzles are used, the former giving lower effective rotational temperatures and more clustering, and the latter giving sharper spectral lines due to reduced Doppler broadening. The pinhole jets are General Valve Series 9 models with orifices in the range of 0.4–0.9 mm. The slit jets are based on a simple modification of the Series 9. The usual O-ring groove on the front face of the jet is filled in and replaced with a larger O-ring groove (15 mm diameter) and a rectangular slot (15 mm × 2 mm × 1 mm). Slit shaped jaws, aligned with the slot, are screwed to the front face and adjusted for the desired opening width. In this way, we obtain a “low-tech,” but still effective, pulsed slit jet nozzle about 14 mm long and 0.05 mm wide. Typical backing pressures in the jet are in the range 1–3 atm, and the pulse repetition rate is typically 4 Hz. The jet opening is operated using a General Valve Iota One controller. 2.2. Laser and optical system The Pb-salt TDL is mounted either in a closed-cycle helium refrigerator (Laser Analytics SP5730) or a liquid nitrogen dewar (L5736). Its output is collected with a collimator assembly (L5120) and passes through a mode selecting monochromator (L5110). Almost all our initial work on CO-containing complexes [3–13] was done using only a single liquid nitrogen cooled diode laser. It had excellent spectral coverage, and was capable of virtually single-mode operation, so that the monochromator was usually bypassed. Following the death of this laser, we were forced to branch out and use the closed cycle refrigerator and monochromator more frequently. After collimation and (possible) mode selection, the laser beam is passed by a series of mirrors to the optical table and ultimately towards the jet chamber. En route, it passes through two pellicle beam splitters which divert small portions of the beam to separate reference gas and etalon wavelength calibration channels. The solid germanium etalon is temperature stabilized, and has a free spectra range of about
0.0162 cm−1 . Also included in the optical path are two iris diaphragms with fixed positions which serve to define the path of the (invisible) infrared radiation. A removable mirror enables the introduction of a coincident He–Ne alignment laser beam. We can also use another He–Ne alignment laser which is built into the L5120 collimator assembly, but it eventually becomes too weak, especially after passing through the mode selecting monochromator. A periscope mirror assembly raises the TDL beam to the height of the jet vacuum chamber and directs it toward the center of the chamber. The beam is passed through the chamber about six to ten times using a Herriott mirror system. These Herriott mirrors are mounted outside the chamber, but losses are minimized by having the CaF2 chamber windows mounted at Brewster’s angle. So far, all our work has been done in the 2050–2450 cm−1 region using three InSb infrared detectors, one each for the supersonic jet, reference gas, and etalon channels. 2.3. Data acquisition electronics and software In order to record a spectrum, the laser is rapidly scanned across a certain wavenumber range (≈0.5–1 cm−1 ) in a 1 ms time interval, using a ramp generated by a Wavetek Model 75 arbitrary waveform generator. The resulting absorption signal from the jet channel detector is digitized at a rate of 2.5 MHz using a Lecroy 9304 digital oscilloscope. Each acquisition cycle consists of jet on, followed by jet off, with subtraction (jet on minus jet off) giving the transient absorption over a flat baseline. A typical spectrum comprises the average of 50–1000 such cycles. Wavenumber calibration is made by simultaneously signal averaging the reference gas and etalon channels, with a calibration scan made automatically after every 25 jet scans. The experiment is controlled by a PC running an operating program written in the Visual Basic language. Timing and data transfer signals to and from the digital oscilloscope, waveform generator, and jet controller are handled by National Instruments (NI) DAQ and GPIB cards mounted in the PC. 2.4. Post experiment data analysis The result of each signal-averaged experimental spectrum is a set of four computer files: one short “header” file containing information supplied by the experimenter together with other relevant parameters for the spectrum, and three data files each containing about 2500 measured points for a particular channel (jet, reference gas, or etalon). Our post experiment data analysis procedures convert this information into a wavenumber-calibrated jet spectrum file. First, we use a commercial analysis package (GRAMS from Thermo Galactic) to measure peak locations in the reference gas file and each fringe in the etalon file. At this stage, any additional desired peaks can be added, and undesired peaks deleted, using commands which are built into
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GRAMS. Our own subroutines written in GRAM’s “Array Basic” language are then used to output files for further processing. The etalon file simply contains the channel number (1–2500) for each etalon fringe peak, while the reference gas file contains the channel number for each desired calibration line together with its known wavenumber, which has been entered on demand by the experimenter. We generally choose from three to eight reference gas lines, which (ideally) are well distributed across the spectrum. Next, our calibration program (written in FORTRAN) reads these latter two files, as well as the original raw jet spectrum file. This program determines the appropriate calibration (i.e. the actual wavenumber as a function of channel number), and outputs the desired calibrated jet spectrum. There are obviously a number of ways in which the calibration could be accomplished within the program. The one we adopted is quite straightforward, and has proven to be robust and accurate. The program uses the chosen reference gas lines and etalon peaks to determine a value for the free spectral range and the “origin” of the fringe pattern. The wavenumber of each point (channel number) in the jet spectrum is then simply determined by linear interpolation between the two closest etalon peaks. This technique avoids any need to fit a polynomial expression to the entire scanned spectrum, and thus adapts well to various degrees of nonlinearity in the scan. Any degradation due to nonlinearity is limited to effects occurring within the (relatively small) free spectral range of about 0.0162 cm−1 . The fitted value for the free spectral range and its standard deviation are output by the program and may be checked by the experimenter in order to evaluate the quality of the resulting calibration. If reference gas lines are scarce in the range of interest, it is also possible to input a known value for the free spectral range and thereby use as few as one reference line. This program also corrects for a small phase shift which occurs between the reference and jet spectra due to the fact that a faster detector/pre-amplifier combination is used in the jet channel. Finally, the calibrated jet spectrum may itself be used as input to GRAMS, which can then automatically measure peak wavenumbers for the observed lines, or used as input to a graph plotting program such as Sigma Plot.
3. Later improvements to the apparatus 3.1. Vacuum system and supersonic jet This part of the apparatus remains similar to the original description. We now have the possibility of using much higher backing pressures and of actively cooling the supersonic jet nozzle, both of which are important for observing the spectra of larger helium clusters described below. The cooling is accomplished by circulating cold methanol (from a refrigerated bath), or cold nitrogen gas (from a liquid nitrogen dewar), through 3 mm. copper tubing which is sol-
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dered to the 6 mm. copper tube which supports the valve. In this way, the valve is conductively cooled by its support, and the gas is pre-cooled on its way to the valve. This setup is mounted inside a 19 mm. thin-walled stainless steel tube in a re-entrant configuration which allows the vacuum feed through and chamber to remain at room temperature, while the valve is cooled. The 19 mm. tube enters the chamber through a bored-out 19 mm. Ultra-Torr fitting. Using a new General Valve Series 99 jet, which has a copper gasket seal, we have recorded spectra with jet temperatures as low as −150 ◦ C and backing pressures up to 50 atm With such high backing densities, the jet repetition rate has to be reduced to 1 Hz or less to avoid overloading the vacuum pumps.
3.2. Laser and optical system One of the most significant improvements to the apparatus has been the installation of a toroidal multi-pass mirror system [8] in place of the original Herriott mirrors. The new system is based on a New Focus Model 5612 cell, subsequently discontinued but now available from Aerodyne Research [29]. It is mounted entirely inside the vacuum chamber on its original base plate, which is rotated in such a way that the spot pattern is oriented with the jet axis (see below) and the base plate does not interfere with the propagation of the jet. In order to fit this assembly, the chamber was lengthened (from 50 cm to 72 cm) in the direction of the optical axis by the addition of two NW250 nipples. In addition to increasing the absorption path through the jet by at least a factor of 10, the new mirror system is also more stable and less prone to optical interference fringes. By altering the normal pattern of laser spots on the cell end mirrors, it is possible to confine the area probed by the laser at the center of the cell (at the jet location) to a rectangle smaller than about 2 cm high×3 cm wide. This gives reasonably good overlap of the probe beam with the jet expansion, particularly when the jet is probed a few centimeter downstream from the nozzle. Although the cell is optimized for 182 passes, it is difficult to be sure that we obtain this value in practice, especially after altering the normal spot pattern. However, by counting spots it is certain that we achieve well over 100 passes. Two precautions are taken to minimize the possible deposition of diffusion pump oil on the mirror surfaces. First, the mirrors are warmed to about 40 ◦ C by means of resistive heaters attached to the mirror mounts. Second, glass tubes (9 cm diameter) are mounted where the body of the Model 5612 cell would normally be located, serving to partly shroud each mirror while still leaving a sufficient gap in the center (16 cm) for the supersonic jet to pass. Other changes in the optical system include more frequent use of the closed-cycle cooler and monochromator, as well as shortening the overall optical path, in part by raising the laser table so that the center line of the vacuum chamber is now only 28 cm above the table surface.
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3.3. Data acquisition electronics and software Significant improvements have been made to the data acquisition system in a number of stages. The resulting scheme is illustrated in Fig. 1 and described in the following paragraphs. The laser signals are now digitized using a 12-bit Gage Compuscope 512 card located in the PC, in place of the external 8-bit Lecroy digital oscilloscope used previously. The Gage card runs at 5 MHz, resulting in the acquisition of 5000 points during our 1 ms scan. Although this represents an over sampling of the data, it has not posed a problem since PC speed, memory, and storage capabilities have more than kept up. Since the Gage card has two input channels, we cannot directly connect our three input signals (jet, reference gas, and etalon). Instead, we connect channel B of the Gage card permanently to the jet signal, and use a simple home-made multiplexer to switch channel A between the reference gas and etalon signals. A control signal from the NI DAQ card switches the multiplexer so that channel A records the reference gas and etalon spectra on alternate scans. Another major change is that we now scan the TDL directly using the ramp modulation function built into its L5830 controller, rather than with the external Wavetek function generator. This means that the laser is always turned on and scanning, and that the initial setup of the laser scan range, typically made using an oscilloscope to observe the reference gas and etalon signals, then applies directly to the data acquisition phase of the experiment. The result is faster and easier experimental setup, together with greater stability and reproducibility. Alternately, a setup function is available in the operating software (we still use Visual Basic) which gives a real-time view of the jet, reference gas, and etalon signals on the computer screen, so that an oscilloscope is not even needed. The PC is connected to the L5830 controller and L5110 monochromator by an RS-232 serial line, so that
experimental variables such as laser current, laser temperature set point, and monochromator wavenumber are automatically recorded in the header file for each spectrum. This serial line could also be used to adjust the L5830 controller parameters from the PC, but we prefer to use the controller’s front panel for this. The timing sequence for data recording is illustrated in Fig. 2. The TDL sweeps continually at a rate of 800 Hz under control of the L5830, and the experimenter sets the laser temperature, current, and modulation amplitude to give the desired wavenumber scan. Data recording is initiated and the program waits for the next laser trigger pulse to be received by the NI DAQ card, as indicated by the vertical arrow in Fig. 2. The program then instructs the Gage card to digitize and save data for 1 ms intervals following each of the next three laser trigger pulses, using its ‘multiple record’ option. It also instructs the NI DAQ card to output a jet trigger signal following a delay time d1 after the next laser trigger pulse. This jet trigger signal goes to the Iota One jet controller, and results in the eventual opening of the jet following a further time delay d2 . Delay d1 is set by the experimenter, whereas delay d2 (≈1 ms) is characteristic of the apparatus and is probably mostly mechanical in nature. While waiting for this delay d2 , the laser continues with another sweep (marked DUMMY in Fig. 2). Then, when the jet is fully open, the next laser sweep is used as the actual jet spectrum. The jet trigger signal is held at a high level until data acquisition is finished in order to minimize electrical pickup in the signal channels. The length of the jet opening (typically 2 ms) is determined by a setting on the front panel of the Iota One, not by the jet trigger signal. After the Gage data acquisition sequence has finished, the program downloads the three data records from the Gage card to the PC memory, discards the middle one (Dummy) and subtracts the first (Background) from the third (Jet Signal) in order to obtain the desired jet spectrum. It also downloads the results from the other channel of the
Fig. 1. A schematic diagram of the experiment control and data acquisition scheme for the pulsed supersonic jet/tunable diode laser apparatus.
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Fig. 2. The timing sequence for data acquisition (see also Fig. 1 and the text).
Gage card, which contains either the reference gas or etalon spectrum, depending on the value of the multiplex control signal (not shown in Fig. 2). The sequence of Fig. 2 is repeated at a rate of 1–4 Hz and the background corrected jet spectrum, the reference gas
spectrum, and the etalon spectrum are signal averaged for the desired number of times (typically 50–1000). The computer screen seen by the experimenter during data acquisition is shown in Fig. 3. The main window at the top of the screen (Longterm Avg) contains the background-corrected,
Fig. 3. The computer screen display for the Visual Basic data acquisition operating program.
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signal-averaged jet spectrum as obtained so far. The middle window (Snapshot) is empty in Fig. 3, but normally contains the jet spectrum from the most recent N scans, where N is the number in the control box “Update every:” The bottom window (etalon/gas cell) contains the average etalon and reference gas spectra. 3.4. Future modifications The modified and refined system as described so far works very well. However, the host PC is beginning to show
its age in terms of speed, memory size, disk storage, and connectivity (e.g. no USB ports). Upgrading only the PC is not so simple, though, since the present NI DAQ and Gage cards are both based on the old ISA bus, which is no longer widely available. Therefore we are currently developing a new system with new PCI bus cards: National Instruments 6024E and Gage Compuscope 1250. Although the new cards are somewhat similar to the old ones, substantial software modifications are still required in order to adapt our program.
Fig. 4. Part of the observed infrared spectra of the weakly-bound 4 He–OCS (lower trace) and 3 He–OCS (upper trace) van der Waals complexes [15,17].
Fig. 5. Spectra taken with increasing backing pressure in a pinhole jet expansion, showing the emergence of the R(0) lines of HeN –OCS clusters [19,21]. The numbers given in circles are equal to N, the number of helium atoms in the cluster.
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Fig. 6. Panoramic view of a series of lines assigned as R(0) transitions of HeN –CO clusters with N = 1–14 [20].
4. Some examples of results The apparatus described here has been used to study many spectra of weakly-bound complexes containing CO (∼2145 cm−1 , or ∼2095 cm−1 for 13 C16 O or 12 C18 O), OCS (∼2060 cm−1 ), N2 O (∼2220 cm−1 ) and CO2 (∼2350 cm−1 ). Quite recently, we were motivated to study the complex He–OCS [15] because of world wide interest in molecular beam studies of helium nanodroplets, which are clusters of 103 –104 helium atoms, possibly incorporating one or a few guest molecules [30,31]. It turns out that OCS is one of the favorite guest molecules: for example, infrared and microwave spectra have been studied extensively for OCS trapped in helium nanodroplets [32–35]. As an example of our results, part of the spectrum of He–OCS is shown in Fig. 4, and more details are available in Refs. [15,17]. During our work on He–OCS, we found that it was also possible to observe spectra due to larger clusters of the form HeN –OCS [19,21], and this has in turn resulted in observations on other cluster systems: HeN –CO [20], HeN –N2 O [22], and HeN –CO2 . An example of the HeN –OCS spectra is shown in Fig. 5. These are pinhole jet traces, stacked in order of increasing backing pressure, with the R(0) (J = 1 ← 0) transitions of the HeN –OCS cluster marked with N, the number of He atoms. Note in Fig. 5 how the lines with N = 2, 3, 4, etc. have little or no intensity in the lowest trace, and then successively emerge as the pressure is increased. These line assignments, up to N = 8, have been confirmed in detail by observations of analogous pure rotational transitions in the microwave region, and they have implications both for the larger helium nanodroplets, and for the nature of superfluidity in finite size systems [19,21]. Even larger clusters are shown in Fig. 6, which shows a panoramic view of R(0) transitions due to HeN –CO clusters with N = 1–14 in the 2145 cm−1 region of the C–O stretch.
Although these assignments have not yet been confirmed by microwave observations or detailed rotational analyses, they are reasonably certain just from the observed spectral pattern and from their dependence on jet backing pressure [20]. The direct observation of transitions of weakly-bound clusters in this size range opens up a new direction in high-resolution infrared spectroscopy.
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[28] M.D. Brookes, D.J. Hughes, B.J. Howard, J. Chem. Phys. 104 (1996) 5391–5405. [29] J.B. McManus, P.L. Kebabian, M.S. Zahniser, Appl. Opt. 34 (1995) 3336–3348. [30] J.P. Toennies, A.F. Vilesov, Ann. Rev. Phys. Chem. 49 (1998) 1–41. [31] Special topic on helium nanodroplets: J. Chem. Phys. 115 (2001) 10065–10281. [32] S. Grebenev, M. Hartmann, M. Havenith, B. Sartakov, J.P. Toennies, A.F. Vilesov, J. Chem. Phys. 112 (2000) 4485–4495. [33] S. Grebenev, J.P. Toennies, A.F. Vilesov, Science 279 (1998) 2083– 2086. [34] S. Grebenev, M. Havenith, F. Madeja, J.P. Toennies, A.F. Vilesov, J. Chem. Phys. 113 (2000) 9060–9066. [35] M. Kunze, P.R.L. Marwick, N. Pörtner, J. Reuss, M. Havenith, J. Chem. Phys. 116 (2002) 7473–7485.
Tunable diode laser spectrometer for pulsed supersonic jets: application to weakly-bound complexes and clusters Matthew D. Brookes, Changhong Xia, Jian Tang, James A. Anstey, Bryan G. Fulsom, Ke-Xian Au Yong, Jenna M. King, A.R.W. McKellar∗ Steacie Institute for Molecular Sciences, National Research Council of Canada, Ottawa Ont., Canada K1A 0R6 Received 4 September 2003; accepted 12 November 2003
Abstract The design and operation of an apparatus for studying infrared spectra of weakly-bound complexes is described in detail. A pulsed supersonic jet expansion is probed using a tunable Pb-salt diode laser spectrometer operated in a rapid-scan mode. The jet may be fitted with either pinhole or slit shaped nozzles, the former giving lower effective rotational temperatures, and the latter giving sharper spectral lines. Notable features of the apparatus include use of a toroidal multi-pass mirror system to give over 100 passes of the laser through the supersonic jet, use of the normal laser controller for laser sweeping during both setup and data acquisition, and use of a simple semi-automated wavenumber calibration procedure. Performance of the apparatus is illustrated with observed spectra of the van der Waals complex He–OCS, and the seeded helium clusters HeN –OCS and HeN –CO. Published by Elsevier B.V. Keywords: Infrared spectra; Spectral lines; Laser spectrometer; Tunable diode laser; Weakly-bound molecules; van der Waals molecules; Data acquisition systems
1. Introduction During the past 20 years, high resolution infrared spectroscopy of weakly-bound van der Waals complexes has emerged as one of the most direct and precise approaches to the measurement of intermolecular forces. In our laboratory, we have studied such spectra with a variety of techniques, for example using gas samples under equilibrium (e.g. cooled long path gas cells) or non-equilibrium (e.g. supersonic jet) conditions, and probing these samples with Fourier transform spectrometers (FTIR) or with tunable laser sources. Our first supersonic jet apparatus utilized a continuous expansion through a slit jet nozzle, as probed by an infrared Pb-salt tunable diode laser (TDL) spectrometer operating in a ‘slow-scan’ (≈0.001 cm−1 s−1 ) mode. Although this gave some excellent results [1,2], it required very large vacuum pumps and consumed very large amounts of sample gas. As an alternative, we started around 1997 to develop a pulsed-jet apparatus using the same TDL spectrometer in a rapid-scan (≈1 cm−1 ms−1 ) mode. Beginning with spectra of the CO
dimer [3], this apparatus has been used to study a number of binary van der Waals complexes [4–18], and, more recently, larger clusters [19–22] containing a number of helium atoms and a single infrared chromophore molecule. The design of this pulsed-jet apparatus and its subsequent evolution is the subject of the present article, together with a sample of some recent results.
2. The original design The first experiment using a tunable infrared laser to study direct infrared absorption spectra of complexes in a pulsed supersonic jet expansion was reported by Hayman et al. [23] in 1985. Many other researchers have subsequently applied this technique, notably Nesbitt and co-workers [24]. Our apparatus was based on the rapid-scan technique introduced by De Piante et al. [25] and also used in a number of other labs [26–28]. 2.1. Vacuum system and supersonic jet
∗
Corresponding author. Tel.: +1-613-990-0736; fax: +1-613-991-2648. E-mail address: [email protected] (A.R.W. McKellar). 1386-1425/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.saa.2003.11.041
The jet chamber is a standard ISO NW250 six-way cross, mounted on a gate valve which in turn is mounted on a
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Varian VHS-10 diffusion pump. This assembly is supported on a 1 in. thick aluminum plate attached to the side of a 1.2×2.4 m optical table, so that the center line of the six-way cross is approximately 43 cm above the table surface. The diffusion pump is backed by Edwards EH500 Roots booster and E2M40 rotary pumps which are located in a separate room at a distance of approximately 4 m. During jet operation, the average gas flow can be conveniently monitored using an ordinary thermocouple vacuum gauge located in the backing line. We use a solenoid-actuated jet which is located entirely inside the vacuum chamber, supported by its 6 mm. gas supply tube. This mounting tube passes through a bored-out 6 mm. Cajon Ultra-Torr fitting in the center of the top plate of the chamber so that the jet fires downward, directly into the diffusion pump. The distance from the jet nozzle to the probe laser beam can be varied by simply raising or lowering the mounting tube. Both pinhole- and slit-shaped jet nozzles are used, the former giving lower effective rotational temperatures and more clustering, and the latter giving sharper spectral lines due to reduced Doppler broadening. The pinhole jets are General Valve Series 9 models with orifices in the range of 0.4–0.9 mm. The slit jets are based on a simple modification of the Series 9. The usual O-ring groove on the front face of the jet is filled in and replaced with a larger O-ring groove (15 mm diameter) and a rectangular slot (15 mm × 2 mm × 1 mm). Slit shaped jaws, aligned with the slot, are screwed to the front face and adjusted for the desired opening width. In this way, we obtain a “low-tech,” but still effective, pulsed slit jet nozzle about 14 mm long and 0.05 mm wide. Typical backing pressures in the jet are in the range 1–3 atm, and the pulse repetition rate is typically 4 Hz. The jet opening is operated using a General Valve Iota One controller. 2.2. Laser and optical system The Pb-salt TDL is mounted either in a closed-cycle helium refrigerator (Laser Analytics SP5730) or a liquid nitrogen dewar (L5736). Its output is collected with a collimator assembly (L5120) and passes through a mode selecting monochromator (L5110). Almost all our initial work on CO-containing complexes [3–13] was done using only a single liquid nitrogen cooled diode laser. It had excellent spectral coverage, and was capable of virtually single-mode operation, so that the monochromator was usually bypassed. Following the death of this laser, we were forced to branch out and use the closed cycle refrigerator and monochromator more frequently. After collimation and (possible) mode selection, the laser beam is passed by a series of mirrors to the optical table and ultimately towards the jet chamber. En route, it passes through two pellicle beam splitters which divert small portions of the beam to separate reference gas and etalon wavelength calibration channels. The solid germanium etalon is temperature stabilized, and has a free spectra range of about
0.0162 cm−1 . Also included in the optical path are two iris diaphragms with fixed positions which serve to define the path of the (invisible) infrared radiation. A removable mirror enables the introduction of a coincident He–Ne alignment laser beam. We can also use another He–Ne alignment laser which is built into the L5120 collimator assembly, but it eventually becomes too weak, especially after passing through the mode selecting monochromator. A periscope mirror assembly raises the TDL beam to the height of the jet vacuum chamber and directs it toward the center of the chamber. The beam is passed through the chamber about six to ten times using a Herriott mirror system. These Herriott mirrors are mounted outside the chamber, but losses are minimized by having the CaF2 chamber windows mounted at Brewster’s angle. So far, all our work has been done in the 2050–2450 cm−1 region using three InSb infrared detectors, one each for the supersonic jet, reference gas, and etalon channels. 2.3. Data acquisition electronics and software In order to record a spectrum, the laser is rapidly scanned across a certain wavenumber range (≈0.5–1 cm−1 ) in a 1 ms time interval, using a ramp generated by a Wavetek Model 75 arbitrary waveform generator. The resulting absorption signal from the jet channel detector is digitized at a rate of 2.5 MHz using a Lecroy 9304 digital oscilloscope. Each acquisition cycle consists of jet on, followed by jet off, with subtraction (jet on minus jet off) giving the transient absorption over a flat baseline. A typical spectrum comprises the average of 50–1000 such cycles. Wavenumber calibration is made by simultaneously signal averaging the reference gas and etalon channels, with a calibration scan made automatically after every 25 jet scans. The experiment is controlled by a PC running an operating program written in the Visual Basic language. Timing and data transfer signals to and from the digital oscilloscope, waveform generator, and jet controller are handled by National Instruments (NI) DAQ and GPIB cards mounted in the PC. 2.4. Post experiment data analysis The result of each signal-averaged experimental spectrum is a set of four computer files: one short “header” file containing information supplied by the experimenter together with other relevant parameters for the spectrum, and three data files each containing about 2500 measured points for a particular channel (jet, reference gas, or etalon). Our post experiment data analysis procedures convert this information into a wavenumber-calibrated jet spectrum file. First, we use a commercial analysis package (GRAMS from Thermo Galactic) to measure peak locations in the reference gas file and each fringe in the etalon file. At this stage, any additional desired peaks can be added, and undesired peaks deleted, using commands which are built into
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GRAMS. Our own subroutines written in GRAM’s “Array Basic” language are then used to output files for further processing. The etalon file simply contains the channel number (1–2500) for each etalon fringe peak, while the reference gas file contains the channel number for each desired calibration line together with its known wavenumber, which has been entered on demand by the experimenter. We generally choose from three to eight reference gas lines, which (ideally) are well distributed across the spectrum. Next, our calibration program (written in FORTRAN) reads these latter two files, as well as the original raw jet spectrum file. This program determines the appropriate calibration (i.e. the actual wavenumber as a function of channel number), and outputs the desired calibrated jet spectrum. There are obviously a number of ways in which the calibration could be accomplished within the program. The one we adopted is quite straightforward, and has proven to be robust and accurate. The program uses the chosen reference gas lines and etalon peaks to determine a value for the free spectral range and the “origin” of the fringe pattern. The wavenumber of each point (channel number) in the jet spectrum is then simply determined by linear interpolation between the two closest etalon peaks. This technique avoids any need to fit a polynomial expression to the entire scanned spectrum, and thus adapts well to various degrees of nonlinearity in the scan. Any degradation due to nonlinearity is limited to effects occurring within the (relatively small) free spectral range of about 0.0162 cm−1 . The fitted value for the free spectral range and its standard deviation are output by the program and may be checked by the experimenter in order to evaluate the quality of the resulting calibration. If reference gas lines are scarce in the range of interest, it is also possible to input a known value for the free spectral range and thereby use as few as one reference line. This program also corrects for a small phase shift which occurs between the reference and jet spectra due to the fact that a faster detector/pre-amplifier combination is used in the jet channel. Finally, the calibrated jet spectrum may itself be used as input to GRAMS, which can then automatically measure peak wavenumbers for the observed lines, or used as input to a graph plotting program such as Sigma Plot.
3. Later improvements to the apparatus 3.1. Vacuum system and supersonic jet This part of the apparatus remains similar to the original description. We now have the possibility of using much higher backing pressures and of actively cooling the supersonic jet nozzle, both of which are important for observing the spectra of larger helium clusters described below. The cooling is accomplished by circulating cold methanol (from a refrigerated bath), or cold nitrogen gas (from a liquid nitrogen dewar), through 3 mm. copper tubing which is sol-
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dered to the 6 mm. copper tube which supports the valve. In this way, the valve is conductively cooled by its support, and the gas is pre-cooled on its way to the valve. This setup is mounted inside a 19 mm. thin-walled stainless steel tube in a re-entrant configuration which allows the vacuum feed through and chamber to remain at room temperature, while the valve is cooled. The 19 mm. tube enters the chamber through a bored-out 19 mm. Ultra-Torr fitting. Using a new General Valve Series 99 jet, which has a copper gasket seal, we have recorded spectra with jet temperatures as low as −150 ◦ C and backing pressures up to 50 atm With such high backing densities, the jet repetition rate has to be reduced to 1 Hz or less to avoid overloading the vacuum pumps.
3.2. Laser and optical system One of the most significant improvements to the apparatus has been the installation of a toroidal multi-pass mirror system [8] in place of the original Herriott mirrors. The new system is based on a New Focus Model 5612 cell, subsequently discontinued but now available from Aerodyne Research [29]. It is mounted entirely inside the vacuum chamber on its original base plate, which is rotated in such a way that the spot pattern is oriented with the jet axis (see below) and the base plate does not interfere with the propagation of the jet. In order to fit this assembly, the chamber was lengthened (from 50 cm to 72 cm) in the direction of the optical axis by the addition of two NW250 nipples. In addition to increasing the absorption path through the jet by at least a factor of 10, the new mirror system is also more stable and less prone to optical interference fringes. By altering the normal pattern of laser spots on the cell end mirrors, it is possible to confine the area probed by the laser at the center of the cell (at the jet location) to a rectangle smaller than about 2 cm high×3 cm wide. This gives reasonably good overlap of the probe beam with the jet expansion, particularly when the jet is probed a few centimeter downstream from the nozzle. Although the cell is optimized for 182 passes, it is difficult to be sure that we obtain this value in practice, especially after altering the normal spot pattern. However, by counting spots it is certain that we achieve well over 100 passes. Two precautions are taken to minimize the possible deposition of diffusion pump oil on the mirror surfaces. First, the mirrors are warmed to about 40 ◦ C by means of resistive heaters attached to the mirror mounts. Second, glass tubes (9 cm diameter) are mounted where the body of the Model 5612 cell would normally be located, serving to partly shroud each mirror while still leaving a sufficient gap in the center (16 cm) for the supersonic jet to pass. Other changes in the optical system include more frequent use of the closed-cycle cooler and monochromator, as well as shortening the overall optical path, in part by raising the laser table so that the center line of the vacuum chamber is now only 28 cm above the table surface.
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3.3. Data acquisition electronics and software Significant improvements have been made to the data acquisition system in a number of stages. The resulting scheme is illustrated in Fig. 1 and described in the following paragraphs. The laser signals are now digitized using a 12-bit Gage Compuscope 512 card located in the PC, in place of the external 8-bit Lecroy digital oscilloscope used previously. The Gage card runs at 5 MHz, resulting in the acquisition of 5000 points during our 1 ms scan. Although this represents an over sampling of the data, it has not posed a problem since PC speed, memory, and storage capabilities have more than kept up. Since the Gage card has two input channels, we cannot directly connect our three input signals (jet, reference gas, and etalon). Instead, we connect channel B of the Gage card permanently to the jet signal, and use a simple home-made multiplexer to switch channel A between the reference gas and etalon signals. A control signal from the NI DAQ card switches the multiplexer so that channel A records the reference gas and etalon spectra on alternate scans. Another major change is that we now scan the TDL directly using the ramp modulation function built into its L5830 controller, rather than with the external Wavetek function generator. This means that the laser is always turned on and scanning, and that the initial setup of the laser scan range, typically made using an oscilloscope to observe the reference gas and etalon signals, then applies directly to the data acquisition phase of the experiment. The result is faster and easier experimental setup, together with greater stability and reproducibility. Alternately, a setup function is available in the operating software (we still use Visual Basic) which gives a real-time view of the jet, reference gas, and etalon signals on the computer screen, so that an oscilloscope is not even needed. The PC is connected to the L5830 controller and L5110 monochromator by an RS-232 serial line, so that
experimental variables such as laser current, laser temperature set point, and monochromator wavenumber are automatically recorded in the header file for each spectrum. This serial line could also be used to adjust the L5830 controller parameters from the PC, but we prefer to use the controller’s front panel for this. The timing sequence for data recording is illustrated in Fig. 2. The TDL sweeps continually at a rate of 800 Hz under control of the L5830, and the experimenter sets the laser temperature, current, and modulation amplitude to give the desired wavenumber scan. Data recording is initiated and the program waits for the next laser trigger pulse to be received by the NI DAQ card, as indicated by the vertical arrow in Fig. 2. The program then instructs the Gage card to digitize and save data for 1 ms intervals following each of the next three laser trigger pulses, using its ‘multiple record’ option. It also instructs the NI DAQ card to output a jet trigger signal following a delay time d1 after the next laser trigger pulse. This jet trigger signal goes to the Iota One jet controller, and results in the eventual opening of the jet following a further time delay d2 . Delay d1 is set by the experimenter, whereas delay d2 (≈1 ms) is characteristic of the apparatus and is probably mostly mechanical in nature. While waiting for this delay d2 , the laser continues with another sweep (marked DUMMY in Fig. 2). Then, when the jet is fully open, the next laser sweep is used as the actual jet spectrum. The jet trigger signal is held at a high level until data acquisition is finished in order to minimize electrical pickup in the signal channels. The length of the jet opening (typically 2 ms) is determined by a setting on the front panel of the Iota One, not by the jet trigger signal. After the Gage data acquisition sequence has finished, the program downloads the three data records from the Gage card to the PC memory, discards the middle one (Dummy) and subtracts the first (Background) from the third (Jet Signal) in order to obtain the desired jet spectrum. It also downloads the results from the other channel of the
Fig. 1. A schematic diagram of the experiment control and data acquisition scheme for the pulsed supersonic jet/tunable diode laser apparatus.
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Fig. 2. The timing sequence for data acquisition (see also Fig. 1 and the text).
Gage card, which contains either the reference gas or etalon spectrum, depending on the value of the multiplex control signal (not shown in Fig. 2). The sequence of Fig. 2 is repeated at a rate of 1–4 Hz and the background corrected jet spectrum, the reference gas
spectrum, and the etalon spectrum are signal averaged for the desired number of times (typically 50–1000). The computer screen seen by the experimenter during data acquisition is shown in Fig. 3. The main window at the top of the screen (Longterm Avg) contains the background-corrected,
Fig. 3. The computer screen display for the Visual Basic data acquisition operating program.
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signal-averaged jet spectrum as obtained so far. The middle window (Snapshot) is empty in Fig. 3, but normally contains the jet spectrum from the most recent N scans, where N is the number in the control box “Update every:” The bottom window (etalon/gas cell) contains the average etalon and reference gas spectra. 3.4. Future modifications The modified and refined system as described so far works very well. However, the host PC is beginning to show
its age in terms of speed, memory size, disk storage, and connectivity (e.g. no USB ports). Upgrading only the PC is not so simple, though, since the present NI DAQ and Gage cards are both based on the old ISA bus, which is no longer widely available. Therefore we are currently developing a new system with new PCI bus cards: National Instruments 6024E and Gage Compuscope 1250. Although the new cards are somewhat similar to the old ones, substantial software modifications are still required in order to adapt our program.
Fig. 4. Part of the observed infrared spectra of the weakly-bound 4 He–OCS (lower trace) and 3 He–OCS (upper trace) van der Waals complexes [15,17].
Fig. 5. Spectra taken with increasing backing pressure in a pinhole jet expansion, showing the emergence of the R(0) lines of HeN –OCS clusters [19,21]. The numbers given in circles are equal to N, the number of helium atoms in the cluster.
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Fig. 6. Panoramic view of a series of lines assigned as R(0) transitions of HeN –CO clusters with N = 1–14 [20].
4. Some examples of results The apparatus described here has been used to study many spectra of weakly-bound complexes containing CO (∼2145 cm−1 , or ∼2095 cm−1 for 13 C16 O or 12 C18 O), OCS (∼2060 cm−1 ), N2 O (∼2220 cm−1 ) and CO2 (∼2350 cm−1 ). Quite recently, we were motivated to study the complex He–OCS [15] because of world wide interest in molecular beam studies of helium nanodroplets, which are clusters of 103 –104 helium atoms, possibly incorporating one or a few guest molecules [30,31]. It turns out that OCS is one of the favorite guest molecules: for example, infrared and microwave spectra have been studied extensively for OCS trapped in helium nanodroplets [32–35]. As an example of our results, part of the spectrum of He–OCS is shown in Fig. 4, and more details are available in Refs. [15,17]. During our work on He–OCS, we found that it was also possible to observe spectra due to larger clusters of the form HeN –OCS [19,21], and this has in turn resulted in observations on other cluster systems: HeN –CO [20], HeN –N2 O [22], and HeN –CO2 . An example of the HeN –OCS spectra is shown in Fig. 5. These are pinhole jet traces, stacked in order of increasing backing pressure, with the R(0) (J = 1 ← 0) transitions of the HeN –OCS cluster marked with N, the number of He atoms. Note in Fig. 5 how the lines with N = 2, 3, 4, etc. have little or no intensity in the lowest trace, and then successively emerge as the pressure is increased. These line assignments, up to N = 8, have been confirmed in detail by observations of analogous pure rotational transitions in the microwave region, and they have implications both for the larger helium nanodroplets, and for the nature of superfluidity in finite size systems [19,21]. Even larger clusters are shown in Fig. 6, which shows a panoramic view of R(0) transitions due to HeN –CO clusters with N = 1–14 in the 2145 cm−1 region of the C–O stretch.
Although these assignments have not yet been confirmed by microwave observations or detailed rotational analyses, they are reasonably certain just from the observed spectral pattern and from their dependence on jet backing pressure [20]. The direct observation of transitions of weakly-bound clusters in this size range opens up a new direction in high-resolution infrared spectroscopy.
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