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Fourier transform infrared jet spectroscopy using a heated slit source
25 September 1998
Chemical Physics Letters 294 Ž1998. 528–532
Fourier transform infrared jet spectroscopy using a heated slit source M. Hepp, F. Herregodts, M. Herman
)
Laboratoire de Chimie Physique Moleculaire CP160r 09, UniÕersite´ Libre de Bruxelles, RooseÕelt AÕe., 50, B-1050 Brussels, Belgium ´ Received 12 February 1998; in final form 29 May 1998
Abstract A supersonic slit-jet apparatus with a large heated slit nozzle apparatus used in combination with a high-resolution Fourier transform infrared spectrometer is described. The production of stable and reproducible expansions of samples, liquid under STP conditions, is demonstrated. The selective observation of vibrational hot bands is also achieved, with gas-phase species. The optimal available spectral resolution of the apparatus is illustrated. q 1998 Published by Elsevier Science B.V. All rights reserved.
1. Introduction As illustrated in the review article by Arno´ and Bevan w1x, the investigation of supersonic expansions with Fourier transform interferometers provides a number of advantages over laser based experiments. The limited resolution and sensitivity are, however, among the major drawbacks of the technique. We attempted to overcome the latter problem at ULB by using a long absorption path slit to increase the signal, and by averaging over a large number of scans to decrease the noise. Our apparatus was already been described in the literature w2x. We illustrate hereafter the optimal resolution it provides. We gained enough sensitivity to study weak overtone bands Že.g., Refs. w2–4x. and unstable molecules w5x. It was, however, achieved at the expense of the
)
Corresponding author. Fax: q32-2-650-4232.
sample consumption. This raises various problems, in particular when investigating samples which are liquid under STP conditions. The standard method in which the vapor is collected from a reservoir using a buffer gas which is bubbled through or blown over the liquid, is indeed no longer appropriate. The high flow rate and large amount of sample required, prevent the equilibrium between liquid and vapor phases being reached in the reservoir, as we experienced w6,7x. As a result, the physical conditions are neither controllable nor reproducible, which is incompatible with the recording times required, first, to tune the conditions and, next, to record the data. We describe in this Letter an apparatus designed to produce stable and reproducible expansions from samples which are liquid under STP conditions. Heating is used to avoid the recondensation of the sample. As a side-effect, the population of vibrationally excited levels is increased and vibrational hot bands can be observed w8x.
0009-2614r98r$ - see front matter q 1998 Published by Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 8 . 0 0 8 8 8 - 4
M. Hepp et al.r Chemical Physics Letters 294 (1998) 528–532
2. Experimental The apparatus, described in Fig. 1, combines a high-resolution Bruker IFS120HR Fourier transform interferometer and a supersonic expansion produced in a 6 l Pyrex cell mounted directly on top of large Roots blowers. The parallel IR beam is coupled out of the interferometer through a KBr window, allowing the interferometer to remain under vacuum. It is focused in the cell with the help of a spherical mirror and then collected by an elliptical mirror onto an external detector. These parts are identical to those described in Ref. w2x and are not detailed further. The distance between the slit and axis of light was ; 6 mm in the present experiments. We determined by experiment the optimal resolution of the apparatus, as described with methane and ethane in Section 3. A new slit nozzle was built, using the procedure described by Pak et al. w9x. It is was, in the present case, made of brass and with two nearly symmetrical pieces forming two slit sections each 7 cm long and 30 mm wide. On each side of the slit, a temperature-controllable heating element Ž150 W
529
each. was incorporated. The temperature, which can be increased up to 2008C, is measured using a thermocouple attached to the slit unit. The heating process is illustrated in Section 5 with the observation of vibrational hot bands in ethane. We developed a specific procedure to deal with liquids, illustrated in Section 4 with methanol, using the apparatus schematized in Fig. 1. The sample is injected with the help of an electromagnetic car fuel injector valve ŽBosch L-Jetronic. into a 3 l copper container which can be heated up to 2008C. The amount of liquid, which is instantaneously evaporated in the hot container, is precisely controlled by the frequency and the pulse duration of the injector. External gas flow controllers allow up to three gases to be mixed with the vapor in the heated volume. At typical total flow rates of 20 lrmin, the mean residence time in the container is ; 10 s. The total and partial vapour pressures can be measured with the help of a Baratron gauge connected on top of the container. The liquid is stored in a separate volume at room temperature and pressed through the injector with the help of a small overpressure of nitrogen. A
Fig. 1. Schematic view of the experimental apparatus.
530
M. Hepp et al.r Chemical Physics Letters 294 (1998) 528–532
ditions are thus obtained, which e.g. allow torsional doublets in ethane w10x to be partly resolved, as illustrated in the bottom part of Fig. 2. The remaining linewidth may arise from various origins, including a residual Doppler effect at the ends of the two slit sections.
4. Investigation of liquid samples We used methanol to test the present apparatus with liquid samples under STP conditions. The spectral region of 3000 cmy1 , recorded using some 80 scans at 0.01 cmy1 resolution and with appropriate optics, is selected here to demonstrate the results in Fig. 3. In addition to recording the spectrum at room temperature Žtop part., for comparison purposes, we
Fig. 2. PŽ2, E . line in n 3 , CH 4 Župper part. and rR 0 Ž1. torsional doublet in n 7 , C 2 H 6 Žlower part., recorded using FT-slit jet spectroscopy at full instrumental resolution.
short, heated tube connects the hot container and the slit nozzle support mounted on the top flange of the vacuum chamber.
3. Optimal spectral resolution We recorded the spectrum of methane and of ethane near 3000 cmy1 , using appropriate optical elements in the interferometer, seeding each gas Ž2.5–3%. in argon Ž150–200 Torr.. The highest possible instrumental resolution was used, i.e. 0.00185 cmy1 Ždefined as 0.9rmaximum optical path difference., with boxcar apodisation. We averaged 12 scans. The linewidth in methane, illustrated in Fig. 2 Žtop part., is measured to be 0.0034 cmy1 ŽFWHM., to be compared to the Doppler width at room temperature Ž0.0091 cmy1 .. Sub-Doppler con-
Fig. 3. Portion of the FT spectrum of CH 3 OH recorded at room temperature Župper part. and under different controlled flow and heated slit-jet conditions Žsee text..
M. Hepp et al.r Chemical Physics Letters 294 (1998) 528–532
performed experiments under two different series of conditions. In the first series Žmiddle part., we used a 1:1 mixing ratio with Ar, with a total stagnation pressure of 670 hPa, at a temperature of 1508C. In the second series Žlower part., the methanol:Ar mixing ratio was decreased to 1:5, the pressure to 230 hPa and the temperature to 708C, i.e. just above the boiling point at atmospheric pressure. For each series, we checked that each block of 10 scans showed similar signal to noise ratios, indicating that the experimental conditions were stable during the whole experiment. The comparison in Fig. 3 of the final, averaged recordings demonstrate the different, now controlled efficiency in the cooling process. This spectral range in methanol was partly studied in the literature w11x. It is likely that the spectral simplification observed in the middle part of Fig. 3, compared to the upper part, results from the partial cooling of the torsion degree of freedom. The spectrum is further simplified in the bottom part, most certainly because of a more efficient cooling affecting the population on both excited torsion levels and high J levels in the ground state. It is hoped that the present results will help in the spectroscopic analysis of this range whose previous investigation, performed with
531
color center lasers in supersonic expansion w11x, suffered from limited spectral coverage.
5. Observation of rotationally cooled hot bands We selected the n 7 transition in ethane to monitor the effect of heating the slit nozzle on the molecules in the expansion. We actually used the whole apparatus shown in Fig. 1, thus including the mixing volume. We performed two series of experiments. In both cases, we recorded 40 scans at an instrumental resolution of 0.01 cmy1 , expanding a 1:1 mixture of ethane and argon, reaching stagnation and residual pressure conditions of 730 and 0.7 hPa, respectively. We heated all elements at 1508C in the first series and not in the second series. A portion of the difference spectrum is presented in Fig. 4. The positive signals turn out to exclusively correspond to lines from the n 7 q n 9 y n 9 hot band, and the negative ones to lines from the n 7 fundamental band. The procedure thus allowed the hot band to be discriminated, whose intensity was increased by heating the apparatus, leading to an overpopulation of the Õ 9 s 1 level. That level is 822 cmy1 above the ground state.
Fig. 4. Portion of the difference spectrum of n 7 , C 2 H 6 , discriminating between hot Žpositive signal. and cold Žnegative signal. bands Žsee text..
532
M. Hepp et al.r Chemical Physics Letters 294 (1998) 528–532
The present result is similar to the one obtained by Oomens and Reuss, using an infrared–infrared laser double resonance procedure w12x. A finer comparison, helped by the spectral portion selected in Fig. 4, which is identical to the upper part of Fig. 3 in the laser investigation w12x, demonstrates that the present data are less resolved and present a lower signal to noise ratio, but show higher selectivity in the discrimination between hot and cold bands. The present heating effect remains quite weak and is really highlighted with the subtraction procedure. We expect hot bands involving vibrational levels above 1500 cmy1 not to be significantly affected, given the maximum temperature available. Vibrations below some 500 cmy1 are expected to behave like rotational degrees of freedom and get cooled down in the expansion, therefore not being affected either.
Acknowledgements The authors wish to thank Professor G. Winnewisser for giving them the possibility of building the new slit nozzle in the workshop of the I. Physikalisches Institut of the University of Cologne and Dr. R. Georges for his help in preparing the new experimental apparatus. MH ŽEC Researcher. acknowledges a postdoctoral position from the EC through the TMR program ŽERBFMBI-CT96-0703.
and FH ŽF.R.I.A. Researcher. a Ph.D. grant from the ‘‘Fonds pour la Formation a` la Recherche dans l’Industrie et dans l’Agriculture’’ ŽF.R.I.A.-Belgium.. The research was supported by the FNRS Žcontract 2.4504.97. and the Communaute´ franc¸aise de Belgique Žcontract ARC 93r98 166..
References w1x J. Arno, ´ J.W. Bevan, in: J.M. Hollas, D. Phillips ŽEds.., Jet Spectroscopy and Molecular Dynamics, ch. 2, Blackie, London, 1995. w2x R. Georges, M. Bach, M. Herman, Mol. Phys. 90 Ž1997. 381. w3x R. Georges, M. Herman, J.-C. Hilico, O. Robert, J. Mol. Spectrosc. 186 Ž1997. in press. w4x M. Hepp, M. Herman, J. Chem. Phys. Ž1998. submitted. w5x R. Georges, J. Lievin, M. Herman, A. Perrin, Chem. Phys. ´ Lett. 256 Ž1996. 675. w6x A. Mellouki, R. Georges, M. Herman, D.L. Snavely, S. Leytner, Chem. Phys. 220 Ž1997. 311. w7x Y. El Youssoufi, R. Georges, J. Lievin, M. Herman, J. Mol. ´ Spectrosc. 186 Ž1997. in press. w8x D. McNaughton, D. McGiverly, E.G. Robertson, J. Chem. Soc., Faraday Trans. 90 Ž1994. 1055. w9x I. Pak, M. Hepp, D.A. Roth, G. Winnewisser, Rev. Sci. Instrum. 68 Ž1997. 1668. w10x A.S. Pine, W.J. Lafferty, J. Res. Natl. Bur. Stand. USA 87 Ž1982. 237. w11x L.-H. Xu, X. Wang, T.J. Cronin, D.S. Perry, G.T. Fraser, A.S. Pine, J. Mol. Spectrosc. 185 Ž1997. 158. w12x J. Oomens, J. Reuss, J. Mol. Spectrosc. 177 Ž1996. 19.
Chemical Physics Letters 294 Ž1998. 528–532
Fourier transform infrared jet spectroscopy using a heated slit source M. Hepp, F. Herregodts, M. Herman
)
Laboratoire de Chimie Physique Moleculaire CP160r 09, UniÕersite´ Libre de Bruxelles, RooseÕelt AÕe., 50, B-1050 Brussels, Belgium ´ Received 12 February 1998; in final form 29 May 1998
Abstract A supersonic slit-jet apparatus with a large heated slit nozzle apparatus used in combination with a high-resolution Fourier transform infrared spectrometer is described. The production of stable and reproducible expansions of samples, liquid under STP conditions, is demonstrated. The selective observation of vibrational hot bands is also achieved, with gas-phase species. The optimal available spectral resolution of the apparatus is illustrated. q 1998 Published by Elsevier Science B.V. All rights reserved.
1. Introduction As illustrated in the review article by Arno´ and Bevan w1x, the investigation of supersonic expansions with Fourier transform interferometers provides a number of advantages over laser based experiments. The limited resolution and sensitivity are, however, among the major drawbacks of the technique. We attempted to overcome the latter problem at ULB by using a long absorption path slit to increase the signal, and by averaging over a large number of scans to decrease the noise. Our apparatus was already been described in the literature w2x. We illustrate hereafter the optimal resolution it provides. We gained enough sensitivity to study weak overtone bands Že.g., Refs. w2–4x. and unstable molecules w5x. It was, however, achieved at the expense of the
)
Corresponding author. Fax: q32-2-650-4232.
sample consumption. This raises various problems, in particular when investigating samples which are liquid under STP conditions. The standard method in which the vapor is collected from a reservoir using a buffer gas which is bubbled through or blown over the liquid, is indeed no longer appropriate. The high flow rate and large amount of sample required, prevent the equilibrium between liquid and vapor phases being reached in the reservoir, as we experienced w6,7x. As a result, the physical conditions are neither controllable nor reproducible, which is incompatible with the recording times required, first, to tune the conditions and, next, to record the data. We describe in this Letter an apparatus designed to produce stable and reproducible expansions from samples which are liquid under STP conditions. Heating is used to avoid the recondensation of the sample. As a side-effect, the population of vibrationally excited levels is increased and vibrational hot bands can be observed w8x.
0009-2614r98r$ - see front matter q 1998 Published by Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 8 . 0 0 8 8 8 - 4
M. Hepp et al.r Chemical Physics Letters 294 (1998) 528–532
2. Experimental The apparatus, described in Fig. 1, combines a high-resolution Bruker IFS120HR Fourier transform interferometer and a supersonic expansion produced in a 6 l Pyrex cell mounted directly on top of large Roots blowers. The parallel IR beam is coupled out of the interferometer through a KBr window, allowing the interferometer to remain under vacuum. It is focused in the cell with the help of a spherical mirror and then collected by an elliptical mirror onto an external detector. These parts are identical to those described in Ref. w2x and are not detailed further. The distance between the slit and axis of light was ; 6 mm in the present experiments. We determined by experiment the optimal resolution of the apparatus, as described with methane and ethane in Section 3. A new slit nozzle was built, using the procedure described by Pak et al. w9x. It is was, in the present case, made of brass and with two nearly symmetrical pieces forming two slit sections each 7 cm long and 30 mm wide. On each side of the slit, a temperature-controllable heating element Ž150 W
529
each. was incorporated. The temperature, which can be increased up to 2008C, is measured using a thermocouple attached to the slit unit. The heating process is illustrated in Section 5 with the observation of vibrational hot bands in ethane. We developed a specific procedure to deal with liquids, illustrated in Section 4 with methanol, using the apparatus schematized in Fig. 1. The sample is injected with the help of an electromagnetic car fuel injector valve ŽBosch L-Jetronic. into a 3 l copper container which can be heated up to 2008C. The amount of liquid, which is instantaneously evaporated in the hot container, is precisely controlled by the frequency and the pulse duration of the injector. External gas flow controllers allow up to three gases to be mixed with the vapor in the heated volume. At typical total flow rates of 20 lrmin, the mean residence time in the container is ; 10 s. The total and partial vapour pressures can be measured with the help of a Baratron gauge connected on top of the container. The liquid is stored in a separate volume at room temperature and pressed through the injector with the help of a small overpressure of nitrogen. A
Fig. 1. Schematic view of the experimental apparatus.
530
M. Hepp et al.r Chemical Physics Letters 294 (1998) 528–532
ditions are thus obtained, which e.g. allow torsional doublets in ethane w10x to be partly resolved, as illustrated in the bottom part of Fig. 2. The remaining linewidth may arise from various origins, including a residual Doppler effect at the ends of the two slit sections.
4. Investigation of liquid samples We used methanol to test the present apparatus with liquid samples under STP conditions. The spectral region of 3000 cmy1 , recorded using some 80 scans at 0.01 cmy1 resolution and with appropriate optics, is selected here to demonstrate the results in Fig. 3. In addition to recording the spectrum at room temperature Žtop part., for comparison purposes, we
Fig. 2. PŽ2, E . line in n 3 , CH 4 Župper part. and rR 0 Ž1. torsional doublet in n 7 , C 2 H 6 Žlower part., recorded using FT-slit jet spectroscopy at full instrumental resolution.
short, heated tube connects the hot container and the slit nozzle support mounted on the top flange of the vacuum chamber.
3. Optimal spectral resolution We recorded the spectrum of methane and of ethane near 3000 cmy1 , using appropriate optical elements in the interferometer, seeding each gas Ž2.5–3%. in argon Ž150–200 Torr.. The highest possible instrumental resolution was used, i.e. 0.00185 cmy1 Ždefined as 0.9rmaximum optical path difference., with boxcar apodisation. We averaged 12 scans. The linewidth in methane, illustrated in Fig. 2 Žtop part., is measured to be 0.0034 cmy1 ŽFWHM., to be compared to the Doppler width at room temperature Ž0.0091 cmy1 .. Sub-Doppler con-
Fig. 3. Portion of the FT spectrum of CH 3 OH recorded at room temperature Župper part. and under different controlled flow and heated slit-jet conditions Žsee text..
M. Hepp et al.r Chemical Physics Letters 294 (1998) 528–532
performed experiments under two different series of conditions. In the first series Žmiddle part., we used a 1:1 mixing ratio with Ar, with a total stagnation pressure of 670 hPa, at a temperature of 1508C. In the second series Žlower part., the methanol:Ar mixing ratio was decreased to 1:5, the pressure to 230 hPa and the temperature to 708C, i.e. just above the boiling point at atmospheric pressure. For each series, we checked that each block of 10 scans showed similar signal to noise ratios, indicating that the experimental conditions were stable during the whole experiment. The comparison in Fig. 3 of the final, averaged recordings demonstrate the different, now controlled efficiency in the cooling process. This spectral range in methanol was partly studied in the literature w11x. It is likely that the spectral simplification observed in the middle part of Fig. 3, compared to the upper part, results from the partial cooling of the torsion degree of freedom. The spectrum is further simplified in the bottom part, most certainly because of a more efficient cooling affecting the population on both excited torsion levels and high J levels in the ground state. It is hoped that the present results will help in the spectroscopic analysis of this range whose previous investigation, performed with
531
color center lasers in supersonic expansion w11x, suffered from limited spectral coverage.
5. Observation of rotationally cooled hot bands We selected the n 7 transition in ethane to monitor the effect of heating the slit nozzle on the molecules in the expansion. We actually used the whole apparatus shown in Fig. 1, thus including the mixing volume. We performed two series of experiments. In both cases, we recorded 40 scans at an instrumental resolution of 0.01 cmy1 , expanding a 1:1 mixture of ethane and argon, reaching stagnation and residual pressure conditions of 730 and 0.7 hPa, respectively. We heated all elements at 1508C in the first series and not in the second series. A portion of the difference spectrum is presented in Fig. 4. The positive signals turn out to exclusively correspond to lines from the n 7 q n 9 y n 9 hot band, and the negative ones to lines from the n 7 fundamental band. The procedure thus allowed the hot band to be discriminated, whose intensity was increased by heating the apparatus, leading to an overpopulation of the Õ 9 s 1 level. That level is 822 cmy1 above the ground state.
Fig. 4. Portion of the difference spectrum of n 7 , C 2 H 6 , discriminating between hot Žpositive signal. and cold Žnegative signal. bands Žsee text..
532
M. Hepp et al.r Chemical Physics Letters 294 (1998) 528–532
The present result is similar to the one obtained by Oomens and Reuss, using an infrared–infrared laser double resonance procedure w12x. A finer comparison, helped by the spectral portion selected in Fig. 4, which is identical to the upper part of Fig. 3 in the laser investigation w12x, demonstrates that the present data are less resolved and present a lower signal to noise ratio, but show higher selectivity in the discrimination between hot and cold bands. The present heating effect remains quite weak and is really highlighted with the subtraction procedure. We expect hot bands involving vibrational levels above 1500 cmy1 not to be significantly affected, given the maximum temperature available. Vibrations below some 500 cmy1 are expected to behave like rotational degrees of freedom and get cooled down in the expansion, therefore not being affected either.
Acknowledgements The authors wish to thank Professor G. Winnewisser for giving them the possibility of building the new slit nozzle in the workshop of the I. Physikalisches Institut of the University of Cologne and Dr. R. Georges for his help in preparing the new experimental apparatus. MH ŽEC Researcher. acknowledges a postdoctoral position from the EC through the TMR program ŽERBFMBI-CT96-0703.
and FH ŽF.R.I.A. Researcher. a Ph.D. grant from the ‘‘Fonds pour la Formation a` la Recherche dans l’Industrie et dans l’Agriculture’’ ŽF.R.I.A.-Belgium.. The research was supported by the FNRS Žcontract 2.4504.97. and the Communaute´ franc¸aise de Belgique Žcontract ARC 93r98 166..
References w1x J. Arno, ´ J.W. Bevan, in: J.M. Hollas, D. Phillips ŽEds.., Jet Spectroscopy and Molecular Dynamics, ch. 2, Blackie, London, 1995. w2x R. Georges, M. Bach, M. Herman, Mol. Phys. 90 Ž1997. 381. w3x R. Georges, M. Herman, J.-C. Hilico, O. Robert, J. Mol. Spectrosc. 186 Ž1997. in press. w4x M. Hepp, M. Herman, J. Chem. Phys. Ž1998. submitted. w5x R. Georges, J. Lievin, M. Herman, A. Perrin, Chem. Phys. ´ Lett. 256 Ž1996. 675. w6x A. Mellouki, R. Georges, M. Herman, D.L. Snavely, S. Leytner, Chem. Phys. 220 Ž1997. 311. w7x Y. El Youssoufi, R. Georges, J. Lievin, M. Herman, J. Mol. ´ Spectrosc. 186 Ž1997. in press. w8x D. McNaughton, D. McGiverly, E.G. Robertson, J. Chem. Soc., Faraday Trans. 90 Ž1994. 1055. w9x I. Pak, M. Hepp, D.A. Roth, G. Winnewisser, Rev. Sci. Instrum. 68 Ž1997. 1668. w10x A.S. Pine, W.J. Lafferty, J. Res. Natl. Bur. Stand. USA 87 Ž1982. 237. w11x L.-H. Xu, X. Wang, T.J. Cronin, D.S. Perry, G.T. Fraser, A.S. Pine, J. Mol. Spectrosc. 185 Ž1997. 158. w12x J. Oomens, J. Reuss, J. Mol. Spectrosc. 177 Ž1996. 19.