- Email: [email protected]
Rotationally resolved overtone spectroscopy of the NO–Ar complex
3 March 2000
Chemical Physics Letters 318 Ž2000. 522–528 www.elsevier.nlrlocatercplett
Rotationally resolved overtone spectroscopy of the NO–Ar complex Y. Kim, K. Patton, J. Fleniken, H. Meyer
)
Department of Physics and Astronomy, UniÕersity of Georgia, Athens, GA 30602-2451, USA Received 20 October 1999; in final form 6 December 1999
Abstract The infrared absorption spectrum of the NO–Ar complex is measured in the region of the first NO overtone band using a new type of IR–UV double resonance technique which combines IR excitation with Ž2 q 1. resonance enhanced multiphoton ionization detection. The long lifetime of the complex Ž0 30 ns. allows the sensitive detection of the vibrationally excited complex itself. Two bands corresponding to D P s 0 and D P s q1 transitions are observed at 3723.4 and 3727.4 cmy1. The rotational analysis is based on a rigid rotor model Hamiltonian and yields the rotational constants AX s 1.795Ž5. cmy1, BX s 0.0785Ž2. cmy1, and CX s 0.0615Ž1. cmy1. The observed splitting of rotational lines is consistent with an increased quenching constant z X s 4.0Ž5. cmy1. q 2000 Elsevier Science B.V. All rights reserved.
1. Introduction In the past, many van der Waals complexes involving closed shell atoms and molecules have been studied at various levels of details w1,2x. Among these, special interest has been devoted to hydrogen bonded systems. On the other hand, only few systems involving open shell molecules have been studied. Most of the spectroscopic work concentrated on complexes involving the hydroxyl radical w3,4x. Another benchmark system for the interaction of a closed shell atom with an open shell diatom is the NO–Ar system. Because of its weak bonding, the main source of experimental information about the
) Corresponding author. Fax: q1 706 542 2492; e-mail: [email protected]
electronic ground state of this system are various scattering experiments. Early experiments concentrated on the measurement of integral and differential cross-sections without state resolution w5–7x. From the energy dependence of the integral cross-section, Thuis et al. deduced a near T-shaped structure of the complex w5x. More recently, state resolved integral and differential cross-section measurements have also been reported w8–12x. Steric effects on state resolved integral cross-sections have been determined by Stolte and coworkers w13x. Theoretically, a first potential surface, based on the electron gas model, was developed by Nielson et al. w14x. The theoretical foundations for studying the dynamics of this system have been formulated by several authors w15–17x. An improved interaction potential was calculated by Alexander using the coupled electron pair approximation w18x. Although this potential surface could reproduce many of the scattering features found ex-
0009-2614r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 0 0 . 0 0 0 5 5 - 5
Y. Kim et al.r Chemical Physics Letters 318 (2000) 522–528
perimentally, there are still significant deviations for the state selective population of parity levels as well as steric effects. Spectroscopically, the complex in its electronic ground state has been studied in the MW region of the electromagnetic spectrum. Mills et al. analyzed the spectrum in terms of an effective rigid rotor Hamiltonian w19,20x. These authors determined the rotational B and C constants as well as the magnitude and the sign of the lambda doubling parameter z w21x. The observed rotational constants are consistent with a 58 deviation from the T-shaped structure of the complex confirming the results of the scattering experiments. On the other hand, the comparison of the spectroscopic results with the calculated bound levels showed significant discrepancies w22x. Stimulated by the existing discrepancies for the scattering and the spectroscopic data, Alexander calculated a new ab-initio potential energy surface very recently w23x. In this contribution, we present first results for the near IR spectroscopy of the NO–Ar complex in its electronic ground state. The spectra associated with the first overtone transition of NO are measured in an IR–UV double resonance experiment. IR transitions of the complex are detected through resonance enhanced multiphoton ionization ŽREMPI. spectroscopy using the NO molecule as a two-photon chromophore. The IR spectra show a large number of rotational lines for the bands characterized by D P s 0 and D P s q1. Obviously, they provide a very sensitive test for the well region of the existing potential surfaces.
2. Experiment The experiments have been performed in a molecular beam scattering apparatus described previously w24,25x. Briefly, the apparatus consists of two differentially pumped chambers for generating and detecting the molecular beam pulses. Pulses of 50 m s duration are generated at a repetition rate of 10 Hz with a homebuilt piezoelectric valve. A mixture of 5% NO in Ar is expanded using a backing pressure of 1.2 bar. The pulses pass from the source chamber through a skimmer into the detection chamber where they are intersected at right angle by two counter-
523
propagating laser beams in the acceleration region of a time-of-flight ŽTOF. mass spectrometer. Tunable UV light in the region around 350 nm is generated by frequency doubling the output of a Nd:YAG laser pumped dye laser system Ž3 GHz linewidth.. The laser beam is focused onto the molecular beam with a 300 mm lens. Pulse energies around 1 mJ are employed. Narrow bandwidth tunable IR radiation Ž450 MHz linewidth, 4 mJ pulse energy. is generated with an OPO laser system ŽCONTINUUM MIRAGE 3000.. The signal and idler output frequencies are separated using two Pellin Broca prisms ŽCaF2 .. NO complexes are excited in the region of the first NO stretch overtone band around 2.7 mm. Because of strong water absorption in this wavelength region, the laser system and the beam path are purged with N2 . The IR beam is focused with a 500 mm lens resulting in a spot size of about 300 mm diameter. The timing between the two laser pulses and the molecular beam pulse is controlled by two digital delay generators. Usually, the UV probe laser is fired 15–30 ns after the IR laser. The IR frequency is monitored with an etalon ŽFSR 0.200918 cmy1 .. Absolute frequency calibration is achieved by simultaneously monitoring the first overtone spectrum of the NO monomer in a photoacoustic cell. Also several water absorption lines in this region have been used for calibration. Because both laser beams must be tightly focused onto the molecular beam, their alignment constitutes the major challenge in this experiment. Beside the spatial and temporal alignment, both lasers need to be tuned to the correct frequencies before a double resonance signal can be found. While the temporal alignment is easily verified using fast photodiodes, the spatial overlap of two laser spots Ž100 mm and 300 mm diameter. under vacuum is a formidable task. In order to reduce the number of unknown parameters, it is extremely important to know the frequencies for the IR transition as well as the two-photon transition. In a first step, we search for a positive Ži.e. background free. double resonance signal for the NO monomer. The IR laser frequency is found by monitoring resonances in a photoacoustic cell. In a next step, the UV laser frequency needs to be tuned to a line which probes the level populated by the IR excitation step. Unfortunately, the UV frequencies for many REMPI detection schemes of
524
Y. Kim et al.r Chemical Physics Letters 318 (2000) 522–528
vibrationally excited molecules are not available. To circumvent these difficulties, we use an electric dc discharge molecular beam source to generate vibrationally excited, but rotationally cold, NO molecules as described in the previous article w26x. With both lasers tuned to the correct frequencies and timed properly, the extreme sensitivity of the ion detection usually provides a small positive double resonance signal, even in the case of a minimal overlap of both beams. The degree of saturation is determined by monitoring the depletion of the initial level in the IR excitation step. The intensity of the focused IR laser beam is sufficient to almost completely saturate rotational transitions of the first overtone band of the NO monomer resulting in depletion values in excess of 40%.
3. Results and discussion Once the alignment of both laser beams has been verified through a positive monomer double resonance signal, IR depletion spectra of the cluster are recorded. In these experiments, the IR laser is scanned while monitoring the Ar–NO Ž ÕXX s 0. complex through a Ž2 q 1. REMPI transition to a vibronic state correlating with the C 2 P Ž ÕX s 2. level of NO. The rovibronic structure of this two-photon transition has been analyzed previously w27x. Fig. 1 shows the resulting NO–Ar depletion spectrum Žtop. together with the room temperature photoacoustic spectrum of the NO monomer Žbottom.. We find a depletion of about 20% for the ŽNOAr.q signal at a frequency of 3723.4 cmy1 . A weaker second band can be identified at 3727.2 cmy1 . In comparison to the NO monomer spectrum, the depletion is substantially reduced because of the larger linewidth of the employed UV laser. The cluster band is red shifted by less than 1.0 cmy1 from the corresponding origin of the monomer band at 3723.85256 cmy1 w28x. The red shift indicates a very small increase in the binding energy and a relatively long lifetime for the vibrationally excited complex w29x. If the lifetime of the excited state is comparable to or larger than the laser pulse duration of 5 ns, it should be possible to detect directly the vibrationally excited complex through Ž2 q 1.REMPI. Therefore, we tune the IR laser frequency to the observed
Fig. 1. IR depletion spectrum of the NO–Ar complex in the vicinity of the origin of the first overtone transition of the NO monomer. At 3724.5 cmy1 the UV laser beam was blocked in order to provide a zero level for the ion signal. The spectrum in the bottom part represents the photoacoustic spectrum of the NO monomer in this region. This spectrum contains a number of lines due to a small water impurity.
depletion feature, and scan the UV laser through the region of the second hot band transition of the E 2 S state of NO. Ž2 q 1. REMPI spectra of this region are shown in Fig. 2. The top spectrum is recorded with the IR laser turned on, while the middle spectrum is recorded without the IR laser. To confirm the assignment as the hot band transition to the E state, we display in the bottom part, the corresponding spectrum for the origin band system shifted by 3723.06 cmy1 w27x. Because both spectra of the E X transition are mainly sensitive to the rovibrational structure in the excited state, it is not surprising that we find identical structures in both spectra. Although the double resonance spectrum in Fig. 2 does not yield any new structural information, the ability to detect the vibrationally excited NO–Ar complex through Ž2 q 1. REMPI, opens the possibility to use REMPI for the sensitive detection of the IR spectrum of the complex.
§
Y. Kim et al.r Chemical Physics Letters 318 (2000) 522–528
525
P- and R-branches. In the top spectrum, the splitting of the R-branch lines is obvious while in the bottom spectrum only one of the two components is detected for each rotational level. The rotational structure of the spectra is analyzed using the rigid rotor Hamiltonian developed by Mills et al. w19x. The model takes into account contributions due to the total angular momentum, the electron orbital angular momentum and its spin. Beside the spin-orbit interaction, it includes also a term describing the quenching of the orbital angular momentum. The resulting effective Hamiltonian is characterized by the three rotational constants A, B, C, the quenching constant z , and the Jacobi bond angle u . The Hamiltonian matrix is set up and diagonalized using basis functions with well defined parity e : < J < P
1
'2
< JP :
qe Ž y . Fig. 2. IR-REMPI double resonance spectrum of NO–Ar. The UV laser is scanned through the region of the hotband transition X XX E 2 SŽ Õ s 0. –X 2 P Ž Õ s 2. of the complex while the IR laser frequency is stabilized onto the maximum depletion signal. The middle spectrum is recorded without the IR laser. Signals in this X XX range are assigned to the cluster transition C 2 P Ž Õ s 2. –X 2 P Ž Õ s 0. w27x. The spectrum in the bottom part represents the Ž2q1. REMPI spectrum of NO–Ar for the excitation from the electronic ground state to its E 2 S-state shifted by 3723.06 cmy1 . Lines marked with an asterisk are caused by a base line shift due to strong monomer resonances at these frequencies.
Fig. 3 displays the IR spectra which were recorded with the UV laser tuned to the bands marked with arrows in Fig. 2. Both spectra show a band around 3723.4 cmy1 characterized by a rich rotational structure. For the spectrum in the top part of Fig. 3, a second band appears in the IR spectrum shifted to the blue by about 4 cmy1 . Obviously, in this case the UV laser is tuned to a transition which is sensitive to a different set of levels populated in the IR transition. These two spectra demonstrate clearly the double resonance character of this experiment. Although the linewidth of the UV laser is relatively large, it is not sufficient to detect with equal probability all levels populated by the IR laser. The low frequency band is characterized by a strong center peak and weaker branches to the red and blue, reminiscent of
lqSqJyPy s
yl S y s : .
< J y P :
Ž 1.
Here J, P refer to the quantum numbers of the total angular momentum and its projection onto the molecule fixed z axis of the complex. The quantum numbers l and s represent the projections of the orbital and spin angular momenta onto the internu-
Fig. 3. IR-REMPI double resonance spectra of NO–Ar. For both spectra, the NO–Ar mass Ž70 amu. is monitored while scanning the IR laser. The REMPI laser is fixed at the indicated UV frequencies, also marked with arrows in Fig. 2.
526
Y. Kim et al.r Chemical Physics Letters 318 (2000) 522–528
clear axis of the diatom. For the calculation of the line strength, we assume that the absorption is associated exclusively with the NO monomer. In the fit of the spectra, we varied the constants AX , BX ,CX , u X , and z X . All other constants were fixed to either their monomer values Ž AXso , AXXso . or to the values determined by Mills et al. for the vibrational ground state. An inspection of the calculated eigenfunctions reveals that the bands observed at 3723.4 cmy1 and 3727.2 cmy1 correspond to excitation of energy levels with a dominant contribution from basis states with P X s 0.5 and P X s 1.5, respectively. So far, we have not been able to find a set of constants which reproduces both bands simultaneously. Therefore, we concentrated our efforts on fitting the low frequency band which involves only the low lying energy levels. For these levels, the rigid rotor model should give a good description. The constants resulting from the fit are listed in Table 1. The spectrum calculated for a rotational temperature of 1.5 K and displayed in Fig. 4 Žspectrum c. is in excellent agreement with the experimental spectrum. The rotationless origin is centered at 3723.06 cmy1 implying a red shift of 0.79 cmy1 . The rotational structure of the low frequency band is sensitive mainly to the constants B and C, while the spacing between the two observed bands is controlled by the constant AX . Since the C constant is associated with the rotation around an axis perpendicular to the plane of the molecule, its value does not depend on the Jacobi angle u w30x. The comparison of the fitted constants with the ones for the vibrational ground state suggests small structural changes when the complex is excited to the first overtone level. The decrease in CX is most likely
Fig. 4. Comparison of calculated spectra with the IR spectra recorded at the two-photon frequencies n˜ s 56727.1 cmy1 Žspectrum a. and n˜ s 56806.1 cmy1 Žspectrum b.. Spectra labeled c through e are calculated using the constants listed in Table 1 and assuming a rotational temperature of 1.5 K. The spectra labeled d and e represent the contributions due to initial levels of negative and positive parity, respectively.
caused by the increase of the distance R Ar – NO , since the NO bond length changes by only 1% upon excitation to ÕX s 2. The fitted value of AX is 8% larger than the rotational constant of the NO monomer, suggesting a substantial deviation from the T shaped structure Ž u s 768.. On the other hand, the fit of the low frequency band yields a value for
Table 1 Spectroscopic constants Žin cmy1 . for the electronic ground state levels of the NO–Ar complex correlating with NO X 2 P Ž Õ s 0,2.. Constants for ArNO Ž Õ s 0. are taken from Ref. w21x X
X
Constant
ArNO Ž Õ s 0.
ArNO Ž Õ s 2, P s 0.5.
Ar–NO Ž Õ s 2, P s 1.5.
n0 A B C z urdeg Durrad A so
– 1.7 0.0760 0.0639 2.687 85.0 0.33 = 10y2 123.1393
3723.06 1.795Ž5. 0.0785Ž2. 0.0615Ž1. 4.0Ž5. 84.7Ž2. 0.33Ž1. = 10y2 122.4606
3727.2 1.785 0.0760 0.0580 4.0 85.4 0.5 = 10y2 122.4606
Y. Kim et al.r Chemical Physics Letters 318 (2000) 522–528
the Jacobi angle of u s 84.78 very similar to the value found for the ground state. This discrepancy reflects the fact that the different contributions to the rigid rotor Hamiltonian depend on different types of averages over the bending angle u . This phenomenon is observed frequently for van der Waals complexes exhibiting large amplitude motion. It is also consistent with the necessity to include centrifugal distortion effects into the Hamiltonian w21x. Because the low frequency band involves transitions between levels with < P < s 0.5, this band must be sensitive to the quenching of the orbital angular momentum of the electrons. This quenching is taken into account by adding a phenomenological term to the Hamiltonian which couples basis states with l s "1 w19,20x. The resulting splitting is clearly seen for the lines of the P- and R-branch in Fig. 4. The fit yields an increased absolute value of the quenching parameter z X with a sign identical to the one for the ground state. Note that the experimental spectrum does not allow to determine the sign of z X
527
independently. According to the assignment of Mills et al., the lowest orbital state of the complex is of AXX symmetry. This result is also in agreement with the energetic ordering of the newly calculated potential surfaces of Alexander w23x. The rotational structure of the spectrum calculated for the blue shifted band agrees to a much lesser extent with the observed spectrum. Although most of the subband positions in Fig. 5 are reproduced, we find clear deviations in the intensity distribution. As the bottom spectrum in Fig. 5 shows, the latter can be improved by increasing the centrifugal distortion constant. Because of the extremely low temperature in the beam, this bands involves mainly D P s q1 transitions originating from levels with P XX s 0.5. In the notation of a symmetric top molecule, these excited levels involve the rotation around the a-axis of the complex. Therefore, the excited levels are much more influenced by large amplitude motion as well as torsional motion or internal rotation of the NO subunit. Both effects must cause a partial breakdown of the rigid rotor model. On the other hand, the position and the rotational structure of this band will provide a very sensitive test for the existing NO–Ar potential energy surfaces in combination with bound state calculations which explicitly take into account the intermolecular bending and stretching coordinates.
4. Conclusion
Fig. 5. Comparison of calculated spectra with the IR spectrum recorded at a two-photon frequency of 56806.1 cmy1 . Spectrum Žb. is calculated using the constants derived for the low frequency X band < P < s 0.5 < and listed in Table 1. For spectrum Žc., we used the increased centrifugal distortion constant DQ s 5.0=10y3 .
We measured the rotationally resolved infrared spectrum of the NO–Ar complex in the region of the first NO stretch overtone using a new type of double resonance technique. In this experiment, pulsed IRexcitation is detected through Ž2 q 1. REMPI of the complex. The rotational analysis suggests only small changes upon vibrational excitation for rotational levels with < P X < s 0.5. The band observed for < P X < s 1.5 could not be reproduced well within the framework of a rigid rotor approximation, most likely because of significant coupling to intermolecular vibrational modes. For the band with < P X < s 0.5, we find an increased quenching constant which suggests a noticeable dependence on the degree of NO stretch vibration.
528
Y. Kim et al.r Chemical Physics Letters 318 (2000) 522–528
Acknowledgements Financial support provided by the National Science Foundation ŽGrant CHE-9707670. is gratefully acknowledged.
References w1x Z. Bacic, R.E. Miller, J. Phys. Chem. 100 Ž1996. 12945. w2x D.J. Nesbitt, Chem. Rev. 88 Ž1988. 843. w3x M.T. Berry, M.R. Brustein, J.R. Adamo, M.I. Lester, J. Phys. Chem. 92 Ž1988. 5551. w4x M.C. Heaven, Annu. Rev. Phys. Chem. 43 Ž1992. 283. w5x H.H. Thuis, S. Stolte, J. Reuss, J.J.H. van den Biesen, C.J.N. van den Meijdenberg, Chem. Phys. 52 Ž1980. 211. w6x M. Keil, J.T. Slankas, A. Kuppermann, J. Chem. Phys. 70 Ž1979. 541. w7x L. Beneventi, P. Casavecchia, G.G. Volpi, J. Chem. Phys. 85 Ž1986. 7011. w8x H. Joswig, P. Andresen, R. Schinke, J. Chem. Phys. 85 Ž1986. 1904. w9x S.D. Jons, J.E. Shirley, M.T. Vonk, C.F. Giese, W.R. Gentry, J. Chem. Phys. 97 Ž1992. 7831. w10x L.S. Bontuyan, A.G. Suits, P.L. Houston, B.J. Whitaker, J. Phys. Chem. 97 Ž1993. 6342.
w11x A. Lin, S. Antonova, A.P. Tsakotellis, G.C. Mcbane, J. Phys. Chem. 103 Ž1999. 1198. w12x M. Islam, I.W.M. Smith, M.H. Alexander, Chem. Phys. Lett. 305 Ž1999. 311. w13x J.J. van Leuken, J. Bulthuis, S. Stolte, J.G. Snijders, Chem. Phys. Lett. 260 Ž1996. 595. w14x G.C. Nielson, G.A. Parker, R.T. Pack, J. Chem. Phys. 66 Ž1977. 1396. w15x S. Green, R.N. Zare, Chem. Phys. 7 Ž1975. 62. w16x M.H. Alexander, J. Chem. Phys. 76 Ž1982. 5974. w17x M.H. Alexander, Chem. Phys. 92 Ž1985. 337. w18x M.H. Alexander, J. Chem. Phys. 99 Ž1993. 7725. w19x P.D.A. Mills, C.M. Western, B.J. Howard, J. Phys. Chem. 90 Ž1986. 3331. w20x W.M. Fawzy, J.T. Hougen, J. Mol. Spectr. 137 Ž1989. 154. w21x P.D.A. Mills, C.M. Western, B.J. Howard, J. Phys. Chem. 90 Ž1986. 4961. w22x T. Schmelz, P. Rosmus, M.H. Alexander, J. Phys. Chem. 98 Ž1994. 1073. w23x M.H. Alexander, J. Chem. Phys. 111 Ž1999. 7426. w24x H. Meyer, J. Chem. Phys. 101 Ž1994. 6686. w25x H. Meyer, J. Chem. Phys. 101 Ž1994. 6697. w26x J. Fleniken, Y. Kim, H. Meyer, Chem. Phys. Lett. 318 Ž2000. 529. w27x H. Meyer, J. Chem. Phys. 107 Ž1997. 7732. w28x C. Amiot, R. Bacis, G. Guelachvili, Can. J. Phys. 56 Ž1987. 251. w29x R.E. Miller, Science 240 Ž1988. 447. w30x J.M. Hutson, in: Advances in Molecular Vibrations and Collision Dynamics, vol. Ia, JAI, 1991, pp. 1–45.
Chemical Physics Letters 318 Ž2000. 522–528 www.elsevier.nlrlocatercplett
Rotationally resolved overtone spectroscopy of the NO–Ar complex Y. Kim, K. Patton, J. Fleniken, H. Meyer
)
Department of Physics and Astronomy, UniÕersity of Georgia, Athens, GA 30602-2451, USA Received 20 October 1999; in final form 6 December 1999
Abstract The infrared absorption spectrum of the NO–Ar complex is measured in the region of the first NO overtone band using a new type of IR–UV double resonance technique which combines IR excitation with Ž2 q 1. resonance enhanced multiphoton ionization detection. The long lifetime of the complex Ž0 30 ns. allows the sensitive detection of the vibrationally excited complex itself. Two bands corresponding to D P s 0 and D P s q1 transitions are observed at 3723.4 and 3727.4 cmy1. The rotational analysis is based on a rigid rotor model Hamiltonian and yields the rotational constants AX s 1.795Ž5. cmy1, BX s 0.0785Ž2. cmy1, and CX s 0.0615Ž1. cmy1. The observed splitting of rotational lines is consistent with an increased quenching constant z X s 4.0Ž5. cmy1. q 2000 Elsevier Science B.V. All rights reserved.
1. Introduction In the past, many van der Waals complexes involving closed shell atoms and molecules have been studied at various levels of details w1,2x. Among these, special interest has been devoted to hydrogen bonded systems. On the other hand, only few systems involving open shell molecules have been studied. Most of the spectroscopic work concentrated on complexes involving the hydroxyl radical w3,4x. Another benchmark system for the interaction of a closed shell atom with an open shell diatom is the NO–Ar system. Because of its weak bonding, the main source of experimental information about the
) Corresponding author. Fax: q1 706 542 2492; e-mail: [email protected]
electronic ground state of this system are various scattering experiments. Early experiments concentrated on the measurement of integral and differential cross-sections without state resolution w5–7x. From the energy dependence of the integral cross-section, Thuis et al. deduced a near T-shaped structure of the complex w5x. More recently, state resolved integral and differential cross-section measurements have also been reported w8–12x. Steric effects on state resolved integral cross-sections have been determined by Stolte and coworkers w13x. Theoretically, a first potential surface, based on the electron gas model, was developed by Nielson et al. w14x. The theoretical foundations for studying the dynamics of this system have been formulated by several authors w15–17x. An improved interaction potential was calculated by Alexander using the coupled electron pair approximation w18x. Although this potential surface could reproduce many of the scattering features found ex-
0009-2614r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 0 0 . 0 0 0 5 5 - 5
Y. Kim et al.r Chemical Physics Letters 318 (2000) 522–528
perimentally, there are still significant deviations for the state selective population of parity levels as well as steric effects. Spectroscopically, the complex in its electronic ground state has been studied in the MW region of the electromagnetic spectrum. Mills et al. analyzed the spectrum in terms of an effective rigid rotor Hamiltonian w19,20x. These authors determined the rotational B and C constants as well as the magnitude and the sign of the lambda doubling parameter z w21x. The observed rotational constants are consistent with a 58 deviation from the T-shaped structure of the complex confirming the results of the scattering experiments. On the other hand, the comparison of the spectroscopic results with the calculated bound levels showed significant discrepancies w22x. Stimulated by the existing discrepancies for the scattering and the spectroscopic data, Alexander calculated a new ab-initio potential energy surface very recently w23x. In this contribution, we present first results for the near IR spectroscopy of the NO–Ar complex in its electronic ground state. The spectra associated with the first overtone transition of NO are measured in an IR–UV double resonance experiment. IR transitions of the complex are detected through resonance enhanced multiphoton ionization ŽREMPI. spectroscopy using the NO molecule as a two-photon chromophore. The IR spectra show a large number of rotational lines for the bands characterized by D P s 0 and D P s q1. Obviously, they provide a very sensitive test for the well region of the existing potential surfaces.
2. Experiment The experiments have been performed in a molecular beam scattering apparatus described previously w24,25x. Briefly, the apparatus consists of two differentially pumped chambers for generating and detecting the molecular beam pulses. Pulses of 50 m s duration are generated at a repetition rate of 10 Hz with a homebuilt piezoelectric valve. A mixture of 5% NO in Ar is expanded using a backing pressure of 1.2 bar. The pulses pass from the source chamber through a skimmer into the detection chamber where they are intersected at right angle by two counter-
523
propagating laser beams in the acceleration region of a time-of-flight ŽTOF. mass spectrometer. Tunable UV light in the region around 350 nm is generated by frequency doubling the output of a Nd:YAG laser pumped dye laser system Ž3 GHz linewidth.. The laser beam is focused onto the molecular beam with a 300 mm lens. Pulse energies around 1 mJ are employed. Narrow bandwidth tunable IR radiation Ž450 MHz linewidth, 4 mJ pulse energy. is generated with an OPO laser system ŽCONTINUUM MIRAGE 3000.. The signal and idler output frequencies are separated using two Pellin Broca prisms ŽCaF2 .. NO complexes are excited in the region of the first NO stretch overtone band around 2.7 mm. Because of strong water absorption in this wavelength region, the laser system and the beam path are purged with N2 . The IR beam is focused with a 500 mm lens resulting in a spot size of about 300 mm diameter. The timing between the two laser pulses and the molecular beam pulse is controlled by two digital delay generators. Usually, the UV probe laser is fired 15–30 ns after the IR laser. The IR frequency is monitored with an etalon ŽFSR 0.200918 cmy1 .. Absolute frequency calibration is achieved by simultaneously monitoring the first overtone spectrum of the NO monomer in a photoacoustic cell. Also several water absorption lines in this region have been used for calibration. Because both laser beams must be tightly focused onto the molecular beam, their alignment constitutes the major challenge in this experiment. Beside the spatial and temporal alignment, both lasers need to be tuned to the correct frequencies before a double resonance signal can be found. While the temporal alignment is easily verified using fast photodiodes, the spatial overlap of two laser spots Ž100 mm and 300 mm diameter. under vacuum is a formidable task. In order to reduce the number of unknown parameters, it is extremely important to know the frequencies for the IR transition as well as the two-photon transition. In a first step, we search for a positive Ži.e. background free. double resonance signal for the NO monomer. The IR laser frequency is found by monitoring resonances in a photoacoustic cell. In a next step, the UV laser frequency needs to be tuned to a line which probes the level populated by the IR excitation step. Unfortunately, the UV frequencies for many REMPI detection schemes of
524
Y. Kim et al.r Chemical Physics Letters 318 (2000) 522–528
vibrationally excited molecules are not available. To circumvent these difficulties, we use an electric dc discharge molecular beam source to generate vibrationally excited, but rotationally cold, NO molecules as described in the previous article w26x. With both lasers tuned to the correct frequencies and timed properly, the extreme sensitivity of the ion detection usually provides a small positive double resonance signal, even in the case of a minimal overlap of both beams. The degree of saturation is determined by monitoring the depletion of the initial level in the IR excitation step. The intensity of the focused IR laser beam is sufficient to almost completely saturate rotational transitions of the first overtone band of the NO monomer resulting in depletion values in excess of 40%.
3. Results and discussion Once the alignment of both laser beams has been verified through a positive monomer double resonance signal, IR depletion spectra of the cluster are recorded. In these experiments, the IR laser is scanned while monitoring the Ar–NO Ž ÕXX s 0. complex through a Ž2 q 1. REMPI transition to a vibronic state correlating with the C 2 P Ž ÕX s 2. level of NO. The rovibronic structure of this two-photon transition has been analyzed previously w27x. Fig. 1 shows the resulting NO–Ar depletion spectrum Žtop. together with the room temperature photoacoustic spectrum of the NO monomer Žbottom.. We find a depletion of about 20% for the ŽNOAr.q signal at a frequency of 3723.4 cmy1 . A weaker second band can be identified at 3727.2 cmy1 . In comparison to the NO monomer spectrum, the depletion is substantially reduced because of the larger linewidth of the employed UV laser. The cluster band is red shifted by less than 1.0 cmy1 from the corresponding origin of the monomer band at 3723.85256 cmy1 w28x. The red shift indicates a very small increase in the binding energy and a relatively long lifetime for the vibrationally excited complex w29x. If the lifetime of the excited state is comparable to or larger than the laser pulse duration of 5 ns, it should be possible to detect directly the vibrationally excited complex through Ž2 q 1.REMPI. Therefore, we tune the IR laser frequency to the observed
Fig. 1. IR depletion spectrum of the NO–Ar complex in the vicinity of the origin of the first overtone transition of the NO monomer. At 3724.5 cmy1 the UV laser beam was blocked in order to provide a zero level for the ion signal. The spectrum in the bottom part represents the photoacoustic spectrum of the NO monomer in this region. This spectrum contains a number of lines due to a small water impurity.
depletion feature, and scan the UV laser through the region of the second hot band transition of the E 2 S state of NO. Ž2 q 1. REMPI spectra of this region are shown in Fig. 2. The top spectrum is recorded with the IR laser turned on, while the middle spectrum is recorded without the IR laser. To confirm the assignment as the hot band transition to the E state, we display in the bottom part, the corresponding spectrum for the origin band system shifted by 3723.06 cmy1 w27x. Because both spectra of the E X transition are mainly sensitive to the rovibrational structure in the excited state, it is not surprising that we find identical structures in both spectra. Although the double resonance spectrum in Fig. 2 does not yield any new structural information, the ability to detect the vibrationally excited NO–Ar complex through Ž2 q 1. REMPI, opens the possibility to use REMPI for the sensitive detection of the IR spectrum of the complex.
§
Y. Kim et al.r Chemical Physics Letters 318 (2000) 522–528
525
P- and R-branches. In the top spectrum, the splitting of the R-branch lines is obvious while in the bottom spectrum only one of the two components is detected for each rotational level. The rotational structure of the spectra is analyzed using the rigid rotor Hamiltonian developed by Mills et al. w19x. The model takes into account contributions due to the total angular momentum, the electron orbital angular momentum and its spin. Beside the spin-orbit interaction, it includes also a term describing the quenching of the orbital angular momentum. The resulting effective Hamiltonian is characterized by the three rotational constants A, B, C, the quenching constant z , and the Jacobi bond angle u . The Hamiltonian matrix is set up and diagonalized using basis functions with well defined parity e : < J < P
1
'2
< JP :
qe Ž y . Fig. 2. IR-REMPI double resonance spectrum of NO–Ar. The UV laser is scanned through the region of the hotband transition X XX E 2 SŽ Õ s 0. –X 2 P Ž Õ s 2. of the complex while the IR laser frequency is stabilized onto the maximum depletion signal. The middle spectrum is recorded without the IR laser. Signals in this X XX range are assigned to the cluster transition C 2 P Ž Õ s 2. –X 2 P Ž Õ s 0. w27x. The spectrum in the bottom part represents the Ž2q1. REMPI spectrum of NO–Ar for the excitation from the electronic ground state to its E 2 S-state shifted by 3723.06 cmy1 . Lines marked with an asterisk are caused by a base line shift due to strong monomer resonances at these frequencies.
Fig. 3 displays the IR spectra which were recorded with the UV laser tuned to the bands marked with arrows in Fig. 2. Both spectra show a band around 3723.4 cmy1 characterized by a rich rotational structure. For the spectrum in the top part of Fig. 3, a second band appears in the IR spectrum shifted to the blue by about 4 cmy1 . Obviously, in this case the UV laser is tuned to a transition which is sensitive to a different set of levels populated in the IR transition. These two spectra demonstrate clearly the double resonance character of this experiment. Although the linewidth of the UV laser is relatively large, it is not sufficient to detect with equal probability all levels populated by the IR laser. The low frequency band is characterized by a strong center peak and weaker branches to the red and blue, reminiscent of
lqSqJyPy s
yl S y s : .
< J y P :
Ž 1.
Here J, P refer to the quantum numbers of the total angular momentum and its projection onto the molecule fixed z axis of the complex. The quantum numbers l and s represent the projections of the orbital and spin angular momenta onto the internu-
Fig. 3. IR-REMPI double resonance spectra of NO–Ar. For both spectra, the NO–Ar mass Ž70 amu. is monitored while scanning the IR laser. The REMPI laser is fixed at the indicated UV frequencies, also marked with arrows in Fig. 2.
526
Y. Kim et al.r Chemical Physics Letters 318 (2000) 522–528
clear axis of the diatom. For the calculation of the line strength, we assume that the absorption is associated exclusively with the NO monomer. In the fit of the spectra, we varied the constants AX , BX ,CX , u X , and z X . All other constants were fixed to either their monomer values Ž AXso , AXXso . or to the values determined by Mills et al. for the vibrational ground state. An inspection of the calculated eigenfunctions reveals that the bands observed at 3723.4 cmy1 and 3727.2 cmy1 correspond to excitation of energy levels with a dominant contribution from basis states with P X s 0.5 and P X s 1.5, respectively. So far, we have not been able to find a set of constants which reproduces both bands simultaneously. Therefore, we concentrated our efforts on fitting the low frequency band which involves only the low lying energy levels. For these levels, the rigid rotor model should give a good description. The constants resulting from the fit are listed in Table 1. The spectrum calculated for a rotational temperature of 1.5 K and displayed in Fig. 4 Žspectrum c. is in excellent agreement with the experimental spectrum. The rotationless origin is centered at 3723.06 cmy1 implying a red shift of 0.79 cmy1 . The rotational structure of the low frequency band is sensitive mainly to the constants B and C, while the spacing between the two observed bands is controlled by the constant AX . Since the C constant is associated with the rotation around an axis perpendicular to the plane of the molecule, its value does not depend on the Jacobi angle u w30x. The comparison of the fitted constants with the ones for the vibrational ground state suggests small structural changes when the complex is excited to the first overtone level. The decrease in CX is most likely
Fig. 4. Comparison of calculated spectra with the IR spectra recorded at the two-photon frequencies n˜ s 56727.1 cmy1 Žspectrum a. and n˜ s 56806.1 cmy1 Žspectrum b.. Spectra labeled c through e are calculated using the constants listed in Table 1 and assuming a rotational temperature of 1.5 K. The spectra labeled d and e represent the contributions due to initial levels of negative and positive parity, respectively.
caused by the increase of the distance R Ar – NO , since the NO bond length changes by only 1% upon excitation to ÕX s 2. The fitted value of AX is 8% larger than the rotational constant of the NO monomer, suggesting a substantial deviation from the T shaped structure Ž u s 768.. On the other hand, the fit of the low frequency band yields a value for
Table 1 Spectroscopic constants Žin cmy1 . for the electronic ground state levels of the NO–Ar complex correlating with NO X 2 P Ž Õ s 0,2.. Constants for ArNO Ž Õ s 0. are taken from Ref. w21x X
X
Constant
ArNO Ž Õ s 0.
ArNO Ž Õ s 2, P s 0.5.
Ar–NO Ž Õ s 2, P s 1.5.
n0 A B C z urdeg Durrad A so
– 1.7 0.0760 0.0639 2.687 85.0 0.33 = 10y2 123.1393
3723.06 1.795Ž5. 0.0785Ž2. 0.0615Ž1. 4.0Ž5. 84.7Ž2. 0.33Ž1. = 10y2 122.4606
3727.2 1.785 0.0760 0.0580 4.0 85.4 0.5 = 10y2 122.4606
Y. Kim et al.r Chemical Physics Letters 318 (2000) 522–528
the Jacobi angle of u s 84.78 very similar to the value found for the ground state. This discrepancy reflects the fact that the different contributions to the rigid rotor Hamiltonian depend on different types of averages over the bending angle u . This phenomenon is observed frequently for van der Waals complexes exhibiting large amplitude motion. It is also consistent with the necessity to include centrifugal distortion effects into the Hamiltonian w21x. Because the low frequency band involves transitions between levels with < P < s 0.5, this band must be sensitive to the quenching of the orbital angular momentum of the electrons. This quenching is taken into account by adding a phenomenological term to the Hamiltonian which couples basis states with l s "1 w19,20x. The resulting splitting is clearly seen for the lines of the P- and R-branch in Fig. 4. The fit yields an increased absolute value of the quenching parameter z X with a sign identical to the one for the ground state. Note that the experimental spectrum does not allow to determine the sign of z X
527
independently. According to the assignment of Mills et al., the lowest orbital state of the complex is of AXX symmetry. This result is also in agreement with the energetic ordering of the newly calculated potential surfaces of Alexander w23x. The rotational structure of the spectrum calculated for the blue shifted band agrees to a much lesser extent with the observed spectrum. Although most of the subband positions in Fig. 5 are reproduced, we find clear deviations in the intensity distribution. As the bottom spectrum in Fig. 5 shows, the latter can be improved by increasing the centrifugal distortion constant. Because of the extremely low temperature in the beam, this bands involves mainly D P s q1 transitions originating from levels with P XX s 0.5. In the notation of a symmetric top molecule, these excited levels involve the rotation around the a-axis of the complex. Therefore, the excited levels are much more influenced by large amplitude motion as well as torsional motion or internal rotation of the NO subunit. Both effects must cause a partial breakdown of the rigid rotor model. On the other hand, the position and the rotational structure of this band will provide a very sensitive test for the existing NO–Ar potential energy surfaces in combination with bound state calculations which explicitly take into account the intermolecular bending and stretching coordinates.
4. Conclusion
Fig. 5. Comparison of calculated spectra with the IR spectrum recorded at a two-photon frequency of 56806.1 cmy1 . Spectrum Žb. is calculated using the constants derived for the low frequency X band < P < s 0.5 < and listed in Table 1. For spectrum Žc., we used the increased centrifugal distortion constant DQ s 5.0=10y3 .
We measured the rotationally resolved infrared spectrum of the NO–Ar complex in the region of the first NO stretch overtone using a new type of double resonance technique. In this experiment, pulsed IRexcitation is detected through Ž2 q 1. REMPI of the complex. The rotational analysis suggests only small changes upon vibrational excitation for rotational levels with < P X < s 0.5. The band observed for < P X < s 1.5 could not be reproduced well within the framework of a rigid rotor approximation, most likely because of significant coupling to intermolecular vibrational modes. For the band with < P X < s 0.5, we find an increased quenching constant which suggests a noticeable dependence on the degree of NO stretch vibration.
528
Y. Kim et al.r Chemical Physics Letters 318 (2000) 522–528
Acknowledgements Financial support provided by the National Science Foundation ŽGrant CHE-9707670. is gratefully acknowledged.
References w1x Z. Bacic, R.E. Miller, J. Phys. Chem. 100 Ž1996. 12945. w2x D.J. Nesbitt, Chem. Rev. 88 Ž1988. 843. w3x M.T. Berry, M.R. Brustein, J.R. Adamo, M.I. Lester, J. Phys. Chem. 92 Ž1988. 5551. w4x M.C. Heaven, Annu. Rev. Phys. Chem. 43 Ž1992. 283. w5x H.H. Thuis, S. Stolte, J. Reuss, J.J.H. van den Biesen, C.J.N. van den Meijdenberg, Chem. Phys. 52 Ž1980. 211. w6x M. Keil, J.T. Slankas, A. Kuppermann, J. Chem. Phys. 70 Ž1979. 541. w7x L. Beneventi, P. Casavecchia, G.G. Volpi, J. Chem. Phys. 85 Ž1986. 7011. w8x H. Joswig, P. Andresen, R. Schinke, J. Chem. Phys. 85 Ž1986. 1904. w9x S.D. Jons, J.E. Shirley, M.T. Vonk, C.F. Giese, W.R. Gentry, J. Chem. Phys. 97 Ž1992. 7831. w10x L.S. Bontuyan, A.G. Suits, P.L. Houston, B.J. Whitaker, J. Phys. Chem. 97 Ž1993. 6342.
w11x A. Lin, S. Antonova, A.P. Tsakotellis, G.C. Mcbane, J. Phys. Chem. 103 Ž1999. 1198. w12x M. Islam, I.W.M. Smith, M.H. Alexander, Chem. Phys. Lett. 305 Ž1999. 311. w13x J.J. van Leuken, J. Bulthuis, S. Stolte, J.G. Snijders, Chem. Phys. Lett. 260 Ž1996. 595. w14x G.C. Nielson, G.A. Parker, R.T. Pack, J. Chem. Phys. 66 Ž1977. 1396. w15x S. Green, R.N. Zare, Chem. Phys. 7 Ž1975. 62. w16x M.H. Alexander, J. Chem. Phys. 76 Ž1982. 5974. w17x M.H. Alexander, Chem. Phys. 92 Ž1985. 337. w18x M.H. Alexander, J. Chem. Phys. 99 Ž1993. 7725. w19x P.D.A. Mills, C.M. Western, B.J. Howard, J. Phys. Chem. 90 Ž1986. 3331. w20x W.M. Fawzy, J.T. Hougen, J. Mol. Spectr. 137 Ž1989. 154. w21x P.D.A. Mills, C.M. Western, B.J. Howard, J. Phys. Chem. 90 Ž1986. 4961. w22x T. Schmelz, P. Rosmus, M.H. Alexander, J. Phys. Chem. 98 Ž1994. 1073. w23x M.H. Alexander, J. Chem. Phys. 111 Ž1999. 7426. w24x H. Meyer, J. Chem. Phys. 101 Ž1994. 6686. w25x H. Meyer, J. Chem. Phys. 101 Ž1994. 6697. w26x J. Fleniken, Y. Kim, H. Meyer, Chem. Phys. Lett. 318 Ž2000. 529. w27x H. Meyer, J. Chem. Phys. 107 Ž1997. 7732. w28x C. Amiot, R. Bacis, G. Guelachvili, Can. J. Phys. 56 Ž1987. 251. w29x R.E. Miller, Science 240 Ž1988. 447. w30x J.M. Hutson, in: Advances in Molecular Vibrations and Collision Dynamics, vol. Ia, JAI, 1991, pp. 1–45.