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On the rotational structure of a prominent band in the vacuum-ultraviolet spectrum of molecular nitrogen
Journal of Electron Spectroscopy and Related Phenomena 151 (2006) 31–33
On the rotational structure of a prominent band in the vacuum-ultraviolet spectrum of molecular nitrogen M. Sommavilla, F. Merkt∗ Laboratorium f¨ur Physikalische Chemie, ETH Z¨urich HCI-H¨onggerberg, CH-8093 Zurich, Switzerland Received 18 August 2005; received in revised form 19 September 2005; accepted 21 September 2005 Available online 22 November 2005
Abstract The rotational structure of the strong band with center around 126412 cm−1 in the vacuum-ultraviolet spectrum of N2 has been elucidated from the temperature dependence of the rotational contour. From the observation of a P(1) transition and the excellent agreement of the rotational contours of the band at rotational temperatures of 8 and 11.5 K with simulations, the symmetry of the upper state of the transition is shown to be 1 + u , in contrast with a recent assignment [H. Lefebvre-Brion, J. Electron Spectrosc. Relat. Phenom. 144–147 (2005) 109]. © 2005 Elsevier B.V. All rights reserved. Keywords: Vacuum-ultraviolet spectroscopy; VUV spectroscopy; High resolution spectroscopy; Photoionization
1. Introduction
basis of:
The region of the absorption and photoionization spectra of molecular nitrogen between 126000 and 126600 cm−1 is characterized by a clustering of transitions to vibronic states of 1 + u symmetry and the almost complete absence of transitions to vibronic states of 1 u symmetry [1]. This clustering poses a challenge for the assignment of the spectrum because it forces one to speculate that the strong band with origin at ∼126412 cm−1 arises as a result of the interaction between the X+ npσ 1 + u, 1 + B+ 3sσ 1 + u and b u states [1], with the additional possibility of a rotational interaction with the A+ (2)4sσ 1 u state as suggested in Ref. [2]. As discussed in Ref. [1], this assignment has several weaknesses and must be regarded as tentative. In particular, the assignment fails to explain why the final state of the transition has not been observed in a multiphoton excitation sequence via the N2 a 1 g (υ = 5) intermediate state [3]. In a recent article [4] and a preliminary report thereof [5], Lefebvre-Brion questions this tentative assignment and proposes that the strong band with origin around 126412 cm−1 be assigned to a final state of 1 u symmetry, the A+ (1)3dσ 1 u state, on the
• the alleged good agreement between the experimental spectrum and a simulation of a transition to an unperturbed final state of 1 u symmetry (see Fig. 5 of Ref. [5]); • coupled-channels calculations of the positions of the high vibrational levels of the b state which predict different spectral positions and different rotational constants for the relevant 1 + states; u • estimates of the autoionization widths of the relevant final states which appear to rule out a final state of 1 + u symmetry; • the value of the rotational constant of the upper level of the transition which is unexpectedly large for a high vibrational level of the b state. We discuss here the details of the rotational analysis of the band of the N2 spectrum with origin around 126412 cm−1 on the basis of which the assignment of the transition to an unperturbed state of 1 u symmetry proposed by Lefebvre-Brion can be ruled out. This analysis motivated our original tentative assignment but was not described in detail in Ref. [1]. 2. Results and discussion
∗
Corresponding author. E-mail address: [email protected] (F. Merkt). URL: http://www.xuv.ethz.ch (F. Merkt).
0368-2048/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2005.09.009
Fig. 1 compares the experimental spectrum of the band at 126412 cm−1 with an array of simulations assuming a final state of 1 u symmetry. The columns in the array are for three
32
M. Sommavilla, F. Merkt / Journal of Electron Spectroscopy and Related Phenomena 151 (2006) 31–33
Fig. 1. Comparison of the photoionization spectrum of N2 in the region of the strong band with origin around 126412 cm−1 with a set of simulations of transitions 1 from the X1 + g ground state to a final state of u symmetry. The temperatures between 6 and 12 K corresponds to the range of temperatures relevant for the experiment, and the rotational constants between 0.7 and 1.9 cm−1 cover the range of plausible values for the final state.
different temperatures of 6, 9 and 12 K covering the range of temperature relevant for the experiment, and the rows correspond to several values of rotational constants between 0.7 and 1.9 cm−1 spanning the range of plausible values. The figure makes it clear that it is impossible to reach a satisfactory agreement with the experimental spectrum assuming that the final state is an unperturbed state of 1 u symmetry, and, indeed, a close look at Fig. 5 of Ref. [5] reveals no better agreement. The sharper structures on the high wavenumber side of the band belong to another transition. From the large difference in the widths of the rotational lines of both transitions one can conclude that the two final states do not interact strongly and that the perturbation of the corresponding rotational structure is not substantial. The photoionization spectra of N2 in the relevant spectral region recorded at two different rotational temperatures of ∼8 and 11.5 K are compared in Fig. 2. The variation of the rota-
tional temperature of the supersonic expansion was achieved by adjusting the nozzle stagnation pressure (details of the experimental procedure can be found in Ref. [1]). At higher temperatures (Fig. 2b), one observes a clear enhancement of the relative intensities of transitions located on the low wavenumber side of the band, which demonstrates that they correspond to transitions out of ground state rotational levels with J ≥ 2. The low wavenumber region of the spectrum reveals a partially resolved rotational structure. Also presented in each panel of Fig. 2 are simulations of the rotational structure of the band assuming a transition to a final state of 1 + u symmetry. The corresponding stick spectra, also shown in Fig. 2, give the positions and intensities of the individual P- and R-branch lines. For the simulations, the band origin and the rotational constant −1 of the 1 + u state were taken to be 126412.4 and 1.61 cm , respectively, and the lineshape was assumed to be Lorentzian with a full width at half maximum of 5.5 cm−1 . The simula-
M. Sommavilla, F. Merkt / Journal of Electron Spectroscopy and Related Phenomena 151 (2006) 31–33
33
tions match the experimental spectra almost perfectly. In particular, the observation of a partially resolved P-branch with a clearly identifiable P(1) line represents a strong argument in favor of an assignment of the final state to a state of 1 + u symmetry. 3. Conclusion Taken together, Figs. 1 and 2 do not represent an absolute proof, only a strong indication, that the final state of the intense transition with origin around 126412 cm−1 is of 1 + u symmetry. It is in principle also conceivable, given the extensive perturbations of the N2 spectrum in the near-threshold region [2], that the final state of the transition is of 1 u symmetry, but sufficiently strongly perturbed that the spectrum accidentally matches the structure of a transition to a 1 + u state. The assignment proposed by Lefebvre-Brion is not compatible with either of these two possibilities. Acknowledgment We thank Dr. H. Lefebvre-Brion for making early versions of her articles available to us prior to publication and for useful discussions. References
Fig. 2. Comparison of the photoionization spectra of N2 (thin lines) in the region of the strong band with origin around 126412 cm−1 with simulations (thick lines) 1 + of transitions from the X1 + g ground state to a final state of u symmetry. The upper panel (a) corresponds to a rotational temperature of 8 K and the lower panel (b) to a temperature of 11.5 K. The stick spectra at the top of each panel give the positions and intensities of the individual P- and R-branch lines.
[1] M. Sommavilla, U. Hollenstein, G.M. Greetham, F. Merkt, J. Phys. B: At. Mol. Opt. Phys. 35 (2002) 3901. [2] Ch. Jungen, K.P. Huber, M. Jungen, G. Stark, J. Chem. Phys. 118 (2003) 4517. [3] E.F. McCormack, S.T. Pratt, J.L. Dehmer, P.M. Dehmer, Phys. Rev. A 42 (1990) 5445. [4] H. Lefebvre-Brion, J. Chem. Phys. 122 (2005) 144315. [5] H. Lefebvre-Brion, J. Electron Spectrosc. Relat. Phenom. 144–147 (2005) 109.
On the rotational structure of a prominent band in the vacuum-ultraviolet spectrum of molecular nitrogen M. Sommavilla, F. Merkt∗ Laboratorium f¨ur Physikalische Chemie, ETH Z¨urich HCI-H¨onggerberg, CH-8093 Zurich, Switzerland Received 18 August 2005; received in revised form 19 September 2005; accepted 21 September 2005 Available online 22 November 2005
Abstract The rotational structure of the strong band with center around 126412 cm−1 in the vacuum-ultraviolet spectrum of N2 has been elucidated from the temperature dependence of the rotational contour. From the observation of a P(1) transition and the excellent agreement of the rotational contours of the band at rotational temperatures of 8 and 11.5 K with simulations, the symmetry of the upper state of the transition is shown to be 1 + u , in contrast with a recent assignment [H. Lefebvre-Brion, J. Electron Spectrosc. Relat. Phenom. 144–147 (2005) 109]. © 2005 Elsevier B.V. All rights reserved. Keywords: Vacuum-ultraviolet spectroscopy; VUV spectroscopy; High resolution spectroscopy; Photoionization
1. Introduction
basis of:
The region of the absorption and photoionization spectra of molecular nitrogen between 126000 and 126600 cm−1 is characterized by a clustering of transitions to vibronic states of 1 + u symmetry and the almost complete absence of transitions to vibronic states of 1 u symmetry [1]. This clustering poses a challenge for the assignment of the spectrum because it forces one to speculate that the strong band with origin at ∼126412 cm−1 arises as a result of the interaction between the X+ npσ 1 + u, 1 + B+ 3sσ 1 + u and b u states [1], with the additional possibility of a rotational interaction with the A+ (2)4sσ 1 u state as suggested in Ref. [2]. As discussed in Ref. [1], this assignment has several weaknesses and must be regarded as tentative. In particular, the assignment fails to explain why the final state of the transition has not been observed in a multiphoton excitation sequence via the N2 a 1 g (υ = 5) intermediate state [3]. In a recent article [4] and a preliminary report thereof [5], Lefebvre-Brion questions this tentative assignment and proposes that the strong band with origin around 126412 cm−1 be assigned to a final state of 1 u symmetry, the A+ (1)3dσ 1 u state, on the
• the alleged good agreement between the experimental spectrum and a simulation of a transition to an unperturbed final state of 1 u symmetry (see Fig. 5 of Ref. [5]); • coupled-channels calculations of the positions of the high vibrational levels of the b state which predict different spectral positions and different rotational constants for the relevant 1 + states; u • estimates of the autoionization widths of the relevant final states which appear to rule out a final state of 1 + u symmetry; • the value of the rotational constant of the upper level of the transition which is unexpectedly large for a high vibrational level of the b state. We discuss here the details of the rotational analysis of the band of the N2 spectrum with origin around 126412 cm−1 on the basis of which the assignment of the transition to an unperturbed state of 1 u symmetry proposed by Lefebvre-Brion can be ruled out. This analysis motivated our original tentative assignment but was not described in detail in Ref. [1]. 2. Results and discussion
∗
Corresponding author. E-mail address: [email protected] (F. Merkt). URL: http://www.xuv.ethz.ch (F. Merkt).
0368-2048/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2005.09.009
Fig. 1 compares the experimental spectrum of the band at 126412 cm−1 with an array of simulations assuming a final state of 1 u symmetry. The columns in the array are for three
32
M. Sommavilla, F. Merkt / Journal of Electron Spectroscopy and Related Phenomena 151 (2006) 31–33
Fig. 1. Comparison of the photoionization spectrum of N2 in the region of the strong band with origin around 126412 cm−1 with a set of simulations of transitions 1 from the X1 + g ground state to a final state of u symmetry. The temperatures between 6 and 12 K corresponds to the range of temperatures relevant for the experiment, and the rotational constants between 0.7 and 1.9 cm−1 cover the range of plausible values for the final state.
different temperatures of 6, 9 and 12 K covering the range of temperature relevant for the experiment, and the rows correspond to several values of rotational constants between 0.7 and 1.9 cm−1 spanning the range of plausible values. The figure makes it clear that it is impossible to reach a satisfactory agreement with the experimental spectrum assuming that the final state is an unperturbed state of 1 u symmetry, and, indeed, a close look at Fig. 5 of Ref. [5] reveals no better agreement. The sharper structures on the high wavenumber side of the band belong to another transition. From the large difference in the widths of the rotational lines of both transitions one can conclude that the two final states do not interact strongly and that the perturbation of the corresponding rotational structure is not substantial. The photoionization spectra of N2 in the relevant spectral region recorded at two different rotational temperatures of ∼8 and 11.5 K are compared in Fig. 2. The variation of the rota-
tional temperature of the supersonic expansion was achieved by adjusting the nozzle stagnation pressure (details of the experimental procedure can be found in Ref. [1]). At higher temperatures (Fig. 2b), one observes a clear enhancement of the relative intensities of transitions located on the low wavenumber side of the band, which demonstrates that they correspond to transitions out of ground state rotational levels with J ≥ 2. The low wavenumber region of the spectrum reveals a partially resolved rotational structure. Also presented in each panel of Fig. 2 are simulations of the rotational structure of the band assuming a transition to a final state of 1 + u symmetry. The corresponding stick spectra, also shown in Fig. 2, give the positions and intensities of the individual P- and R-branch lines. For the simulations, the band origin and the rotational constant −1 of the 1 + u state were taken to be 126412.4 and 1.61 cm , respectively, and the lineshape was assumed to be Lorentzian with a full width at half maximum of 5.5 cm−1 . The simula-
M. Sommavilla, F. Merkt / Journal of Electron Spectroscopy and Related Phenomena 151 (2006) 31–33
33
tions match the experimental spectra almost perfectly. In particular, the observation of a partially resolved P-branch with a clearly identifiable P(1) line represents a strong argument in favor of an assignment of the final state to a state of 1 + u symmetry. 3. Conclusion Taken together, Figs. 1 and 2 do not represent an absolute proof, only a strong indication, that the final state of the intense transition with origin around 126412 cm−1 is of 1 + u symmetry. It is in principle also conceivable, given the extensive perturbations of the N2 spectrum in the near-threshold region [2], that the final state of the transition is of 1 u symmetry, but sufficiently strongly perturbed that the spectrum accidentally matches the structure of a transition to a 1 + u state. The assignment proposed by Lefebvre-Brion is not compatible with either of these two possibilities. Acknowledgment We thank Dr. H. Lefebvre-Brion for making early versions of her articles available to us prior to publication and for useful discussions. References
Fig. 2. Comparison of the photoionization spectra of N2 (thin lines) in the region of the strong band with origin around 126412 cm−1 with simulations (thick lines) 1 + of transitions from the X1 + g ground state to a final state of u symmetry. The upper panel (a) corresponds to a rotational temperature of 8 K and the lower panel (b) to a temperature of 11.5 K. The stick spectra at the top of each panel give the positions and intensities of the individual P- and R-branch lines.
[1] M. Sommavilla, U. Hollenstein, G.M. Greetham, F. Merkt, J. Phys. B: At. Mol. Opt. Phys. 35 (2002) 3901. [2] Ch. Jungen, K.P. Huber, M. Jungen, G. Stark, J. Chem. Phys. 118 (2003) 4517. [3] E.F. McCormack, S.T. Pratt, J.L. Dehmer, P.M. Dehmer, Phys. Rev. A 42 (1990) 5445. [4] H. Lefebvre-Brion, J. Chem. Phys. 122 (2005) 144315. [5] H. Lefebvre-Brion, J. Electron Spectrosc. Relat. Phenom. 144–147 (2005) 109.