High resolution cw diode laser cavity ring down spectroscopy in hydrogen plasma at room temperature

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1 High resolution cw diode laser cavity ring down spectroscopy in hydrogen plasma at room temperature P. Macko ), R. Plašil, P. Kudrna, P. Hlavenka, V. Poterya, A. Pysanenko, G. Bánó ) J. Glosík Charles University in Prague, Faculty of Mathematics and Physics, Department of Electronics and Vacuum Physics, V Holesovickach 2, Praha 8, Czech Republic Received 6 June 2002 Reported is a description of an infrared cavity ring down (CRD) spectrometer employing a cw diode laser, that is applied for absorption measurements in a microwave discharge. The apparatus was recently built for study of recombination of H + 3 (v = 0) with electrons. In cavity ring down spectroscopy (CRDS) the monochromatic light of the laser is coupled into an optical cavity composed of high reflectivity mirrors. Interrupting the laser beam the light intensity within the cavity decays due to losses on the mirrors and also due to absorbtion by gas phase molecules (and/or molecular ions). The characteristic time constant of the exponential decay is coupled with the absorption coefficient, which is given by the corresponding absorption cross section and by the density of absorbing species. Using the CRDS method the absolute density of H + 3 ions in hydrogen containing plasma is determined. The sensibility of the new CRDS system (< cm 1 ) was found to be sufficient for this kind of measurements. PACS: Dx, Fi Key words: H3+, laser spectroscopy 1 Introduction Electron ion recombination is an extremely important ionisation loss process in natural plasmas including interstellar gas clouds and planetary ionospheres, and in man made plasmas such as laser plasmas, combustion flames and plasmas used in technologies. In hydrogen containing plasmas the recombination of H + 3 ions plays a key role. H + 3 ions were detected in interstellar clouds and in atmospheres of Jupiter, Saturn, Uranus and Neptune (see e.g. review by A. Dalgarno [1]). Despite the fact that recombination of H + 3 ions with electrons is one of the most studied recombination processes, both experimentally and theoretically, the agreement between theory, experiment and astronomical observations was not achieved yet. Experimentally obtained recombination rate coefficients differ by two orders of magnitude (see compilation in review by R. Plašil et al. [2], and paper by A. Pysanenko et al. [3] in present volume ). Theoretical predictions of the recombination rate coefficient of H + 3 (v=0) are giving values < 10 8 cm 3 s 1 (see recent paper by V. Kokoouline et ) permanent address: Comenius University, Faculty of Mathematics, Physics and Informatics, Department of Plasma Physics, Mlynská dolina F2, , Bratislava ) permanent address: Research Institute for Solid State Physics and Optics, Hungarian Academy of Sciences, Konkoly Thege u , 1121 Budapest Czechoslovak Journal of Physics, Vol. 52 (2002), Suppl. D D695

2 P. Macko et al. al. [4]). In our recent studies we have measured this recombination rate coefficient in an afterglow experiment in He/Ar/H 2 mixture. We found that the dissociative recombination of H + 3 ions with electrons at thermal energies is a very slow process with rate coefficient < cm 3 s 1. We have also observed that in an afterglow plasma in He/Ar/H 2 mixture the recombination of H + 3 dependens on the partial pressure of hydrogen. This indicates that the observed recombination is a three body process [2, 5, 6, 7]. The mentioned experimental studies were carried out in Advanced Integrated Stationary Afterglow (AISA) using Langmuir probe and mass spectrometer, i.e. without identification of internal excitation of recombining ions. The decay time of the plasma in AISA is very long (up to 80 ms) and recombining ions are most probably internally relaxed, nevertheless direct identification of the internal state of recombining ions will be usefull. Because there are experimental evidences that the recombination of H + 3 is dependent on internal excitation (see e.g. compilation in ref.[2]) we decided to identify the recombining ions by means of CRDS. Infrared absorption spectroscopy was used for study of the recombination of H + 3 already by Amano [8, 9] and by Fehér [10]. Their measurements were carried out in the early afterglow of a pure hydrogen discharge at pressures of Torr. The final goal of the present study is to measure the recombination coefficient in the mixture of He/Ar/H 2 with H 2 partial pressures in the range of Torr, to confirm observation of the above mentioned pressure dependence of the recombination rate coefficient [2, 5, 6, 7]. Time resolved vibrational absorption spectroscopy can be used to obtain the evolution of H + 3 ion number density during the afterglow, which is required for studies of the reaction kinetics of these ions. Figure 1 shows a one dimensional version of the potential energy surface of H + 3 given by Röhse et al. [11]. Until now, several vibrational transitions were studied. Figure 2 shows the band centres and relative intensities of the strongest vibrational bands at an assumed temperature of 600 K. The bars at the bottom of the diagram show the spectral ranges that have been continuously scanned by absorption spectroscopy. The absorption on the overtone band ν 2 =3 0 (observed at around 1.4 µm) is suitable for H + 3 ion density measurements. The absorption coefficient of the strongest line of this band (assuming a H + 3 (v=0) density of 1010 cm 3 and temperature of 300 K) is about 10 7 cm 1. The Cavity Ring Down Spectroscopy has been introduced in 1988 by O Keefe and Deacon [13] as a technique allowing high sensitivity absorption measurements using pulsed laser sources [14, 15, 16]. Due to their large amplitude fluctuations these lasers were otherwise to remain useless for absorption spectroscopy. Even if pulsed laser have the great advantage of a large spectral coverage (NIR to UV), they still remain expensive laboratory instruments. This limits the application of CRDS in domains such as trace detection, where compact and portable devices are needed. More recently, cw CRDS technique was introduced functioning with CW single frequency lasers [17, 18, 19, 20, 21, 22]. The high sensitivity of CRDS is due to the enhancement of the effective optical pathlength when laser light coupled into an optical cavity is reflected back and forth by the high reflection mirrors of the cavity. The technique is based on the D696 Czech. J. Phys. 52 (2002)

3 High resolution cavity ring down spectroscopy... Fig. 1. Vibrational energie levels of H + 3 where two H H bond lengths are required to be equal, and the angle between them θ is varied [11]. The arrow shows the overtone band ν 2=3 0 observed at around 1.4 µm. measurement of the decay of the light intensity leaking from one of the mirrors when a laser light is injected into the cavity through the other mirror. For linear absorption, this decay is exponential with decay time τ(ν), given by: 1 τ(ν) = 1 + d cα(ν) (1) τ 0 L where τ 0 is the decay time in the absence of any absorption, d and α(ν) are the length and absorption coefficient of the absorbing medium, respectively. L is the length of the cavity. CRDS is a powerful technique, well adapted for the measurement of very weak absorption signals. It was used for detection of ionised states [14], of low densities of N 2 molecules vibrationally excited to the 18 th level [15, 16], overtone transition in HCN molecule in visible spectral range [23] or Herzberg bands of O 2 in UV [24]. This technique can be used to measure the transition probability [25], the concentration using very weak transition [26, 27], the absolute concentration of complexes [28] or radicals [29, 30, 31], for monitoring plasma processes [32, 33, 34], to measure the concentration of negative ions [35, 36], or to study the kinetics of chemical reactions [37]. The detection limit of the CRDS method is 10 9 cm 1, which is commonly achieved by mirrors having reflection of %. This value is two orders of magnitude lower than the predicted absorption coefficient of the ν 2 =3 0 H + 3 band. In the present paper a new cavity ring down spectrometer employing a cw diode Czech. J. Phys. 52 (2002) D697

4 P. Macko et al. Fig. 2. Vibrational band relative intensities of H + 3 in absorption at 600 K [12]. laser is described in detail, which was built for study of formation and recombination of H + 3 ions in a Stationary Afterglow (SA). 2 Experimental set up The experimental set up of the new cw CRDS is shown in Figure 3. An external cavity single mode tuneable diode laser (Sacher Lasertechnik), having output power of about 3 mw, is used. The output surface of the laser diode is antireflection coated that suppresses the inside modes of the diode and prefers the mode of the external resonator. The laser wavelength is tuned by the external cavity in Littman configuration (total tuning range is actually only from 1457 nm to 1480 nm). The reduction is probably du to the degradation of the antireflection coating. To avoid the re injection of the reflected beam from the input mirror of the ringdown cavity into the diode laser, an optical isolator is used. After passing through the isolator, part of the laser beam is split off. Part of it passes through a Fabry Perot interferometer with free spectral range of 3.33 GHz, which gives the precise frequency shift calibration. The principal laser beam enters an acousto optic modulator (AOM) and a space filter composed of two lenses and a pinhole of 50 µm. A lens (focal length of 100 mm) focuses the laser beam to the pinhole. A diaphragm, placed after the pinhole, selects the 0 th mode from the diffraction image after the pinhole. The next lens (focal length of 50 mm) matches the beam, having the free D698 Czech. J. Phys. 52 (2002)

5 High resolution cavity ring down spectroscopy mode "0" mode "1" 1 Optical isolator LASER Fabry- Pérot interfero metr PC beam splitter Fig. 3. Scheme of the CW CRDS set up: 1 beamsplitter, 2 InGaAs photodiode, 3 acousto optical modulator, 4 tilting mirrors, 5 lenses, 6 pinhole, 7 piezoelectric transducer, 8 super dielectric mirrors, 9 avalanche InGaAs photodiode. 8 space paraxial Gaussian like profile, to the TEM 00 cavity mode. The cavity of 710 mm length is composed by two supermirrors of reflectivity % around 1470 nm (Layertek). The mirrors have radius of curvature of 100 cm and one of them is mounted on a piezoelectric transducer in order to modulate the cavity length. This way the laser frequency is periodically matched to one of the TEM 00 modes. The ringdown signal, leaking from one of the cavity mirrors, is detected by an avalanche InGaAs photodiode, amplified by a transimpedance amplifier (500 kω, bandpass up to 1 MHz), digitised by a 14 bits 2 MHz PC card (ADLink 2010) and recorded in a PC. To observe clean ringdown decays, the laser beam is interrupted by the AOM once the transmitted signal of TEM 00 mode goes above a given threshold. Laser frequency is scanned by a signal from the D/A converter of ADLink The same PC card detects and stores the signal from the Fabry Perot interferometer during each CRD event. The microwave plasma is created in the cavity placed at the central region of the discharge tube (made of Pyrex, with inner diameter of 4 cm). The estimated effective length of the discharge column is about 5 cm. To prevent the diffusion of active species from the discharge to the mirrors, the discharge volume is separated from the mirror mounts by diaphragms of 6 mm diameter. (see. Fig. 4). Further suppression of an eventual plasma enhanced deposition on the surface of the mirror is achieved by a flow of gas (He) entering the discharge tube close to the mirrors and flowing towards the discharge region. The gas flow is maintained by a mass flow controller and by pumping through a throttle valve. Czech. J. Phys. 52 (2002) D699

6 P. Macko et al. gas inlet plasma pumping gas inlet Fig. 4. Schema of discharge tube with tilting mirror mounts. 3 Results and discussion The new system was first tested on the residual H 2 O vapours remaining in the discharge chamber. Before the measurements the apparatus was evacuated to 1 Torr. The laser wavelength was tuned to the region of the ν 2 =3 0 H + 3 band (around 6805 cm 1 ) and was scanned through the absorption lines of the H 2 O. The obtained spectrum is presented in Figure 5. For comparison the tabulated spectrum (taken from HITRAN 96 database) is shown as well. It can be seen that saturation of the detection occurs at position of the strongest lines. Measurements of the H + 3 (v=0) ion were made with a microwave discharge operating in He/Ar/H 2 mixture. Our previous calculations and measurements indicated that in such mixtures the highest H + 3 density occures during the discharge afterglow. In order to find the highest signal, the discharge was operated both in continuous and pulsed regime. In the pulsed regime (when we also take the advantage of the afterglow period) 0.5 ms microwave pulses were used to generate the plasma. The time between pulses was varied in the range of 2 6 ms. To optimise the H + 3 (v=0) D700 Czech. J. Phys. 52 (2002)

7 High resolution cavity ring down spectroscopy α (10-6 cm -1 ) wavenumber (cm -1 ) Fig. 5. Spectrum containing absorption lines of H 2O around 6805 cm 1. density during the afterglow the evolution of ionic composition was calculated starting when the plasma is switched off. The details of the calculation and the used rate coefficients of the considered processes are described in our previous papers in connection with recombination studies of the H + 3 in stationary afterglow AISA experiment [2, 5, 6, 7]. As a result we obtained the optimal ratio of the different buffer gas components, for which the H + 3 density is the highest. During the measurements the data aquisition was carried out continuously (without any synchronization) regardless of the discharge regime (pulsed or continuous). It follows, that in pulsed regime, the scattering of the ring down decay times (at the absorption wavelength) will indicate the variation of the H + 3 (v=0) density during the discharge and the afterglow period. We found, that keeping the length of the afterglow below 2.5 ms, no pronounced scattering of the signal occures. We conclude that for these conditions a significant H + 3 (v=0) density is present in the afterglow and also during the active discharge time. In Figure 6 two examples of the obtained spectra are plotted. All the points represent an average of 20 ring down measurements. The measured FWHM of the absorption lines (1.8 GHz) corresponds to the Doppler broadening at about 300 K. It can be seen that the detection limit (given by the noise of the signal) is below the 2x10 8 cm 1 level. The signal to noise ratio can be further improved by making the absorption column longer. It follows, that the new experimental setup reported here is suitable for H + 3 density measurements in the decaying He/Ar/H 2 plasmas. Time resolved measurements are planned for the near future. Czech. J. Phys. 52 (2002) D701

8 P. Macko et al. 3.0x10-7 continuous regime pulsed regime 2.5x x10-7 α (cm -1 ) 1.5x x x x vawenumber (cm -1 ) Fig. 6. Spectrum of the He/Ar/H 2 microwave discharge. The ratio of the components was kept (380/1/0.5) at overall pressure of 8 mbar. The average microwave power of 43 and 36 W was applied for the pulsed and continuous regime, respectively. The length of the afterglow period was adjusted to be 2.5ms in the pulsed discharge measurement. The arrows indicate two absorption lines of H + 3 (v=0). Thanks for financial support are due to GACR (202/99/D061, 205/02/0610, 202/02/0948), GAUK (146/2000 B FYZ MFF) and MSM The experiments were carried out with support from EC s RTN under contract HPRN CT , ETR and with support from Euroatom. P. M. and G. B. are thankfull to EC for financial support. References [1] A. Dalgarno, Advances in atomic, molecular, and optical physics 32 (1994) 57 [2] R. Plašil, J. Glosík, V. Poterya, P. Kudrna, J. Rusz, M. Tichý, A. Pysanenko, Int. J. Mass Spectrometry, (2002) in print [3] A. Pysanenko, O. Novotný, P. Zakouřil, R. Plašil, V. Poterya, J. Glosík, Cz.J. Physics, (2002) present volume. [4] V. Kokoouline, Chris H. Greene and B.D.Esry, Nature 412 (2001) 891 [5] J. Glosík, R. Plašil, V. Poterya, P. Kudrna, M. Tichý, Chem. Phys. Letters 331 (2000) 209 [6] J. Glosík, R. Plašil, V. Poterya, P. Kudrna, M. Tichý and A. Pysanenko, J. Phys. B: At. Mol. Opt. Phys. 34 (2001) L485 D702 Czech. J. Phys. 52 (2002)

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