Rotational-resolved pulsed field ionization photoelectron study of NO a 3,v 0 16 in the energy range of ev

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1 JOURNAL OF CHEMICAL PHYSICS VOLUME 111, NUMBER 5 1 AUGUST 1999 Rotational-resolved pulsed field ionization photoelectron study of NO a 3,v 0 16 in the energy range of ev G. K. Jarvis Chemical Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 4720 Y. Song and C. Y. Ng a) Ames Laboratory, USDOE and Department of Chemistry, Iowa State University, Ames, Iowa Received 28 March 1999; accepted 22 April 1999 We have obtained rotationally resolved pulsed field ionization photoelectron PFI-PE spectra of NO in the energy range of ev, covering ionization transitions of NO (a 3,v 0 16,J ) NO(X 2 3/2,1/2,v0,J). The PFI-PE bands for NO (a 3,v 1 5,7 10,12 14,16) obtained in this experiment represent the first rotationally resolved spectroscopic data for these states. The simulation of these PFI-PE bands provides accurate molecular constants for NO (a 3,v 0 5,7 10,12 14,16), including ionization energies, vibrational constants e cm 1, e e cm 1, and rotational constants B e cm 1, e cm 1. As observed in the PFI-PE study of NO (X 1 ), this experiment reveals a generally increasing trend for the maximum J value and intensities of higher J branches as v or bond distance for NO (a 3 ) is increased. This observation can be taken as strong support of the electron-molecular-ion-core scattering model for angular momentum and energy exchanges in the threshold photoionization of NO American Institute of Physics. S I. INTRODUCTION The experiments were carried out at the Chemical Dynamics Beamline 4 of the ALS associated with the Lawrence Berkeley National Laboratory. The beamline consists of a 10-cm period undulator, a gas harmonic filter, a 6.65-m offplane Eagle mounted monochromator, and a multipurpose photoelectron-photoion spectrometer, all of which have been described in detail previously. 1 6 In the present experiment, Ne was used in the gas filter, suppressing higher undulator harmonics with photon energies greater than ev. Undulator light of the first harmonic emerging from the gas filter was directed into the monochromator and dispersed by a 2400-lines/mm grating (dispersion0.64 Å/mm) before entering the photoelectrona Author to whom correspondence should be addressed; electronic mail: cyng@ameslab.gov Recently, we have developed a new pulsed field ionization photoelectron PFI-PE scheme using high-resolution monochromatized undulator synchrotron radiation at the Chemical Dynamics Beamline of the Advanced Light Source ALS. 1 6 In this synchrotron-based technique, 1 high-n (n 100) Rydberg states are field ionized by a small electric field pulse applied in the dark gap of a synchrotron ring period. Using an electron spectrometer equipped with a steradiancy analyzer and a hemispherical energy analyzer arranged in tandem, we have shown that PFI-PEs can be detected with little prompt electron background for a delay of only 8 ns with respect to the beginning of the dark gap. 1,2,5 Most recently, we have further developed a time-of-flight TOF selection scheme for PFI-PE detection. 7 The achieved resolutions in these synchrotron-based techniques are shown to be similar to those of vacuum ultraviolet VUV laser studies. 1,7 However, the energy range of synchrotron study is far greater 6 30 ev than that 19 ev of VUV laser sources. 8 The most attractive feature of a synchrotron source is its ease of tunability, making rotationally resolved PFI-PE measurements for many molecules a routine operation. Employing our synchrotron-based PFI-PE techniques we have performed an extensive photoelectron study on the ground state of NO, rotationally resolving the vibrational progression for NO (X 1,v 0 32). 9 This is the second in a series of papers on NO where the NO a 3, v 0 5, 7 10, 12 14, and 16 states are rotationally analyzed in the energy range of ev. To our knowledge, no rotational-resolved spectroscopic data for NO (a 3 ) have been reported previously. The previous HeI photoelectron study of Edquivst et al. 10 has revealed a vibrational progression for NO (a 3,v ), providing information on the vibrational constants up to v 8 for this state. Rotational data that have been available to date have been based on interpolation of data for CO and N 2 by Field. 11 Therefore, the present rotationally resolved study is of direct importance from a spectroscopic point of view. By simulating the PFI-PE NO (a 3,v ) spectra using the Buckingham Orr Sichel BOS model, 12 we have obtained accurate ionization energy IE values, rotational constants, and vibrational constants for these v levels. We note that the rotational and vibrational constants determined here can be used to construct an accurate potential energy curve for NO (a 3 ). II. EXPERIMENT /99/111(5)/1937/10/$ American Institute of Physics

2 1938 J. Chem. Phys., Vol. 111, No. 5, 1 August 1999 Jarvis, Song, and Ng photoion apparatus. Monochromator entrance/exit slits of 50/50 m nominal wavelength resolution0.032 Å full width at half maximum FWHM were used for all PFI-PE vibrational bands presented here, except for the v 9PFI-PE bands, where 100/100-m slits nominal wavelength resolution0.064 Å FWHM were used. The NO sample was introduced as an effusive beam through a metal orifice with a diameter of 0.5 mm at room temperature and a distance of 0.5 cm from the photoionization/photoexcitation PI/PEX region. We estimate that the NO density in the PI/PEX region is 10 3 Torr and that the rotational temperature for the NO sample is 298 K. The procedures for synchrotron-based PFI-PE measurements have been described in detail previously. 1,2 In the present experiment, a nominally zero electrostatic direct current dc field was maintained across the PI/PEX region prior to the application of a pulsed electric field height 1.2 V/cm, width40 ns to the repeller plate on the electron spectrometer side of the PI/PEX region. The pulsed electric field was applied with a period identical to the synchrotron ring period of 656 ns or frequency of 1.53 MHz and a delay corresponding to 20 ns after the beginning of the 80-ns dark gap. Counting times for specific spectra varied between 2 and 30 s per point, depending on the intensity of the band under observation. Spectra were flux normalized using the photon signal obtained at a tungsten detector positioned behind the PI/PEX region intercepting the monochromatized VUV beam. The VUV detection efficiency of the tungsten detector as a function of photon energy was corrected for using the known photoelectric yield curve. 13 All spectra were calibrated before and after each experiment using the Kr ( 2 P 3/2 ), Ar ( 2 P 3/2 ), and Ne ( 2 P 3/2 ) PFI-PE peaks obtained under the same experimental conditions. 2,4,11 This calibration scheme assumes that the Stark shifts for the IEs of NO and the rare gases are identical. Previous measurements by us indicate that the accuracy of this calibration method is within 0.5 mev. 2,3,5,8,14 Pulsed field ionization photoelectron spectra for NO (a 3,v 9) were recorded using the TOF PFI-PE detection scheme as described in Jarvis et al. 7 This scheme is both more sensitive and cleaner in terms of hot electron rejection than our previous method, 1 whereby a hemispherical analyzer was used for hot electron suppression. Spectra from the two detection methods were normalized to each other based on the measured intensities of the NO (a 3,v 9) and NO (X 1,v ) bands appearing in this energy region. 9 III. RESULTS AND DISCUSSION The ground NO(X 2 ) state has the main electronic configuration, 1 2 2* * * 1. Including the spin-orbit interaction, NO(X 2 1/2 ) becomes the ground electronic state. The excited spin-orbit NO( 2 3/2 ) state is known to locate cm 1 above the ground NO(X 2 1/2 ) state. The NO (a 3 ) ionic state results from the removal of one of the electrons from the 1 bonding orbital. 15 A. Relative PFI-PE vibrational band intensities for NO a 3,v 0 16 Figure 1 shows the PFI-PE spectrum for NO in the energy range of ev. The positions of the NO (a 3,v 0 21) bands are marked in the figure, along with the positions of vibrational bands for the NO (b 3, w 3, b 3, A 1, and W 1 electronic states. Complete analysis of these latter states will be reserved for later publications in this series. As can be seen from Fig. 1, the NO (a 3,v 16) vibrational band is the last clearly observable band for this electronic state. The bands at NO a 3, v 6, 11, 15, 17, and 18 are obscured by overlapping states. The NO (a 3,v 19) bands are not observed within the sensitivity of our instrument. The relative intensities of the NO a 3, v 0 5, 7 10, 12 14, and 16 PFI-PE bands are listed in Table I. The PFI-PE band for NO (a 3,v 1) is found to have overwhelmingly the maximum intensity arbitrarily normalized to a value of 100 in the table. All other PFI-PE vibrational bands are at least ten times smaller in intensity than this single vibrational band, indicating that some types of resonant enhancement mechanism may be operative for the NO (a 3,v 1) PFI-PE band. No other clear maxima are observed and, in general, the intensity of the bands decreases as v increases. In the previous HeI photoelectron studies of Edquivst et al., 10 Natalis et al., 16 and Collin et al., 17 the vibrational maximum was found to be at v 3 and the vibrational progression extends to v 9. Edquivst et al.actually only observed up to v 8. The length of the progression observed here extends seven vibrational bands higher then these earlier HeI studies. Since the mechanisms for the formation of NO (a 3,v ) in the HeI experiments and the present PFI-PE measurements are different, we expect that the relative vibrational band intensities observed in HeI ionization are different from those in PFI-PE detection. The intensities for the PFI-PE bands of NO (a 3,v 0 16) observed here are most likely mediated by near-resonance autoionization Rydberg states 18 and/or repulsive 19,20 neutral states. We have selected to show the rotationally resolved PFI-PE bands NO a 3, v 0 5, 7, and 9 in Figs. 2a 2h upper spectra, open circles, respectively. All spectra are normalized to reflect their relative observed intensities in the ordinate. Since the PFI-PE bands for NO (a 3, v 8, 10 14, and 16 are similar to those for NO (a 3, v 7 and 9, they are not shown here. As shown in Fig. 1, the PFI-PE bands for NO a 3, v 12 and 16 have finite overlap with the low energy tail of the stronger bands for NO (W 1,v 3) and NO (b 3,v 1), respectively. The rotational levels for NO(X 2 1/2,3/2 ) and NO (a 3 ) are labeled by the total angular momentum quantum numbers J and J, respectively. The J value for the NO(X 2 1/2 )NO( 2 3/2 ) state is equal to N1/2N3/2, where N is the rotational

3 J. Chem. Phys., Vol. 111, No. 5, 1 August 1999 Photoelectron study of NO 1939 FIG. 1. PFI-PE spectrum of NO in the range ev showing the relative intensities and positions for the NO (a 3,v 0 21) states. Note that the NO (a 3,v 6,11,15,17 21) are either unresolved due to overlap with other states or too weak to be observed. Also marked in the figure are vibrational states for the NO b 3, w 3, b 3, A 1,andW 1 electronic states that are accessible in this region. quantum number for NO. For NO (a 3 ),J is identical to the rotational quantum number N. Thus, for transitions originating from NO(X 2 1/2 )NO( 2 3/2 ), NN N J1/2J3/2. Each rotational level in NO(X 2 )is split into the / parity levels. Since this small -doubling is not resolved in the present experiment, individual J levels can be viewed as being doubly degenerate. As a result, transitions from a J level to all J levels are allowed. B. Simulation of rotational transition intensities The relative intensities for rotational structures observed in individual vibrational bands for NO a 3, v 0 5, 7 10, 12 14, and 16 were simulated using the BOS model, 12 which was derived to predict rotational line strengths observed in single-photon ionization of diatomic molecules. This procedure has been described by us previously, 21,22 and a detailed description of the specifics of this simulation can be found in our first paper 9 on the NO (X 1 ) state which undergoes a similar to transition in its formation Hund s case a to b. 23 Note that the spin-rotation splittings of NO (a 3 ) have been neglected in the present simulation as they are likely to be small compared to the resolution of our PFI-PE data. The BOS coefficients, C, (1 4), used in the simulation for PFI-PE bands for NO a 3, v 0 5, 7 10, 12 14, and 16 are listed in Table II. For the transition between a and a state, 1, the first 3 j symbol of Eq. 2 of Ref. 9 requires 1, i.e., the C 0 term is zero. As noted in our first paper, 9 possible J transitions are JJ Jl3/2,l1/2,...,l3/2, where l is the orbital angular momentum quantum number of the ejected photoelectron. The 3/2 term of Eq. 1 can be thought of as the addition of the spin angular momentum 1/2 for the photoelectron and the angular momentum of the photon 1. The simulation uses known spectroscopy constants, 24 e ( ), e e (14.100), B e ( ), and e ( cm 1 for NO(X 2 1/2 )NO( 2 3/2 ). Using these constants, the BOS simulation allows the determination of accurate rotational constants, B v, and IE values for the formation of NO (a 3, v 0 5, 7 10, 12 14, and 16, J from NO(X 2 1/2,3/2,v0,J). The simulated spectra lower spectra, solid circles are compared to the experimental PFI-PE bands for NO a 3, v 0 5, 7 and 9 in Figs. 2a 2h, respectively. The rotational temperature used for NO in the simulated spectra 1

4 1940 J. Chem. Phys., Vol. 111, No. 5, 1 August 1999 Jarvis, Song, and Ng TABLE I. Ionization energies IEs and relative intensities for the transitions NO (a 3, v 0 16, N 0) NO(X 2 1/2, v0, J1/2) and vibrational spacings and rotational constants (B v ) for NO (a 3, v 0 16). IE ev B v cm 1 Relative intensity ev v PFI-PE a,b Ref. 26 c mev d PFI-PE a,b Ref. 26 c cm 1 e PFI-PE a,f PFI-PE a,b a This work. Estimated uncertainties in the last few significant figures are given in brackets immediately following the table value for IE and for B v values. Uncertainties are 10% for the relative PFI-PE intensities of v 0 9 and 20% for those of v 10 to 16. b The best-fitted values calculated using the Dunham coefficients see Table III are given in parentheses underneath the experimental values. c Values calculated using Dunham coefficients of Ref. 26. d IEPFI-PEIE Ref. 26, where IEPFI-PE values are obtained in the present study and IE Ref. 26 values are calculated using Dunham coefficients of Ref. 26. e B v (PFI-PE)B v Ref. 26, where B v PFI-PE values are obtained in the present study and B v Ref. 26 values are calculated using Dunham coefficients of Ref. 26. f The PFI-PE intensity at v 1 is arbitrarily normalized to 100. was 298 K. The positions of the rotational transitions for ionization transitions NO (a 3,v,J ) NO(X 2 1/2,v0,J) and NO (a 3,v,J ) NO(X 2 3/2,v0,J) are marked as up-pointing and down-pointing lines, respectively, in these figures. Because of the large amount of details in the figures, identifications of the markings with J J values are not made. For individual J branches, the positions of the first and second transitions e.g., J 0 J1/2 and J 1 J3/2 for J1/2 associated with transitions from NO(X 2 1/2 ) are indicated by progressively shorter lines, thus revealing the origin and direction of the rotational branches. For v 0 to 3 J9/2 to 7/2 and for v 4 to 16 J 11/2 to 7/2 transitions are shown in the figures. The J9/2 and 11/2 transitions are not marked as they are not as clearly identifiable as their positive counterparts. The rotational branches resolved in the PFI-PE bands for NO (a 3, v 0 5, 7 10, 12 14, and 16 are all found to shade to the red with the branch heads appearing on the high-energy side of the spectra. As observed in the PFI-PE study of the NO (X 1 ) state, 9 an increase in the angular momentum exchanged between the photoelectron and the ion core is also observed here as v increases. For NO (a 3,v 0 3), the rotational branches contributing to the spectra are J1/2, 3/2, 5/2, 7/2, and 9/2, which correspond to the for-

5 J. Chem. Phys., Vol. 111, No. 5, 1 August 1999 Photoelectron study of NO 1941 FIG. 2. Comparison of experimental and simulated PFI-PE spectra for NO (a 3 ): a v 0, b v 1, c v 2, d v 3, e v 4, f v 5, g v 7, and h v 9. All spectra were measured at a nominal wavelength resolution of Å full width at half maximum FWHM. The positions of the rotational transitions for NO(X 2 1/2 ) NO (a 3 ) and NO(X 2 3/2 ) NO (a 3 ) are also shown as up-pointing and down-pointing lines, respectively. In each J case shown, the position of the first and second transitions e.g., J 0J1/2 and J 1J3/2 for J1/2 are indicated by progressively shorter lines.

6 1942 J. Chem. Phys., Vol. 111, No. 5, 1 August 1999 Jarvis, Song, and Ng FIG. 2. Continued.

7 J. Chem. Phys., Vol. 111, No. 5, 1 August 1999 Photoelectron study of NO 1943 FIG. 2. Continued.

8 1944 J. Chem. Phys., Vol. 111, No. 5, 1 August 1999 Jarvis, Song, and Ng FIG. 2. Continued.

9 J. Chem. Phys., Vol. 111, No. 5, 1 August 1999 Photoelectron study of NO 1945 TABLE II. Best-fitted BOS coefficients obtained in the simulation of the PFI-PE bands for NO (a 3, v 0 16). The sum of the C values is normalized to 1.00 and identical C values were used for 2 1/2 and 2 3/2 transitions. V BOS coefficients a C 1 C 2 C 3 C a All C values have an uncertainty of 0.2. mation of continuum photoelectron angular momentum states l0, 1, 2, 3, and 4. The J11/2 band head is clearly observed in the PFI-PE bands for NO a 3, v 4 and 5 see Figs. 2e and 2f. In order to obtain good fits to the experimental spectra for NO a 3, v 4 and 5, C 4 has to be greater than zero see Table II. The nonzero C 4 values for NO (a 3,v 4) are indicative of a finite contribution of the J11/2 rotational branch to the experimental PFI-PE spectra for these higher v states. The observation of J1/2, 3/2, 5/2, 7/2, 9/2, and 11/2 rotation branches in the spectra for NO a 3, v 4, 5, 7 10, 12 14, and 16 indicates that the continuum photoelectron states are l0, 1, 2, 3, 4, and 5. The angular momentum exchange between the outgoing photoelectron and the NO core may be considered to result from collisions between the outgoing electron and the NO ion core. The theoretical study shows that the angular momentum coupling between l partial waves is induced by the torques associated with the nonspherical nature of the NO molecular ion core potential, making a photoelectron orbital an admixture of angular momentum components. 25 Since the equilibrium bond distance for NO increases as v is increased, the NO ion core potential is thus more anisotropic at higher v states. The increase in the angular momentum transfer most likely reflects the increase in bond distances of NO in higher v states, which causes an increase in the inelastic cross sections for higher angular momentum transfers in electron-molecular ion core collisions. Changes in angular momentum may be more gradual than are indicated in Table II. However, the BOS model is not all that sensitive at the measured resolution to changes in C of 0.2, and hence small fluctuations cannot be discerned. For the NO a 3, v 0, 2 4, 7 and 9 PFI-PE bands see Figs. 2a, 2c 2e, 2g, and 2h, respectively, excellent agreement between the simulated and experimental TABLE III. Comparison of vibrational and rotational constants for NO (a 3 ) determined here and in Ref. 26. Dunham constants This work cm 1 Ref. 26 cm 1 T e e e x e B e e data is obtained. For the NO (a 3,v 1) PFI-PE band Fig. 2b, however, a large perturbation is observed originating from either the J3/2 or 5/2 band head. It is unlikely that this comes about from overlap with another direct-ionization state as no resulting vibrational progression is observed. As shown in Fig. 1, the NO (X 1,v 28) vibrational band does lie on the high-energy side of this transition, but is too far removed to cause this effect. The perturbation is therefore most likely caused by the presence of low-n Rydberg states. 18 For the NO (a 3,v 5) PFI-PE band Fig. 2f, the whole of the NO( 2 3/2 ) transition is found to be enhanced relative to that of the NO(X 2 1/2 ) transitions. This phenomenon is again most likely caused by overlap with low-n Rydberg interloper states. 18 The observation of a long vibrational progression for NO (a 3 ) with a relatively smooth PFI-PE band intensity profile is also consistent with the direct excitation model. 19 This model invokes an intermediate neutral repulsive state, which has finite couplings to long-lived high-n Rydberg states converging to ionization thresholds of NO (a 3,v,J ). Due to the repulsive nature of the intermediate neutral state, no pronounced local intensity enhancements are expected in the PFI-PE spectrum. We believe that both the nearby resonance autoionizing mechanisms 18 and the direct excitation mechanism 19 are likely to be operative in PFI-PE measurements 9 such as this. As expected, the intensity enhancement due to the nearby resonance autoionizing mechanism 18 is usually significantly greater and more local than the direct 19 excitation mechanism. C. Ionization energies and spectroscopic constants On the basis of the BOS simulation, accurate IE and B v values for the NO a 3, v 0 5, 7 10, 12 14, and 16 vibrational states have been determined and are listed in Table I. The IE values for the vibrational states correspond to energies for ionization transitions NO (X 1,v,J 0) NO(X 2 1/2,v0,J1/2). Vibrational and rotational constants obtained were fitted to the standard equations to obtain the vibrational and rotational Dunham coefficients e, e e, e e, and e z e and B e, e, and e. 23,24 These coefficients obtained by a least-squares fit are listed in Table III along with those determined by Albritton et al. 26 We note that the latter determination is based only on data for v 9. The IE values for v 0 16 calculated using the Dunham coefficients of Ref. 26 are included in Table I to compare with experimental values determined in this study. The deviations, see Table

10 1946 J. Chem. Phys., Vol. 111, No. 5, 1 August 1999 Jarvis, Song, and Ng I between the corresponding experimental IE values determined here and calculated values indicate that the calculated IE values for lower v states are less accurate varying up to 7.3 mev at v 0 than those for higher v states. This deviation is most likely caused by the poorer resolution of the previous photoelectron data 10,16 upon which the vibrational Dunham coefficients were derived. 26 The B v values for v 0 16 calculated using the Dunham coefficients obtained by Albritton et al. 26 are also compared to B v values determined in the present study in Table I. The rotational Dunham coefficients of Ref. 26 are based only on data for v 9 and these come from an interpolation technique devised by Field for CO and N 2 to calculate rotational constants from the corresponding vibrational data. 10 As shown by the deviations () in Table I, the B v values predicted by the rotational Dunham coefficients of Ref. 26 are generally too high by cm 1 compared to corresponding experimental values for all v. The interpolation technique used by Albritton does in fact give incorrect B v and e values when applied to the NO (X 1 ) state for which rotational constants have been determined experimentally by ourselves 9 and by emission studies. 11,27 So it is perhaps no surprise that there is also disagreement here. We have included in Table I the best-fitted IE and B v values based on the Dunham coefficients see Table III determined in the present study. The absolute IE discrepancies between the corresponding PFI-PE and best-fitted values for all v states are 1 mev except those for v 12 16, the absolute discrepancies of which are in the range of mev. The absolute discrepancies between experimental and best-fitted B v values are less than cm 1. IV. CONCLUSIONS We have obtained rotationally resolved PFI-PE spectra for NO in the region of ev using monochromatized synchrotron radiation at the Chemical Dynamics Beamline of the ALS. This measurement gives unambiguous identifications of the NO a 3, v 0 5, 7 10, 12 14, and 16 bands. The rotational-resolved PFI-PE data for NO (a 3 ) have allowed the experimental determination of accurate rotational constants for this state for the first time. Local deviations observed between the experimental and BOS simulated intensities for rotational transitions are attributed to perturbations by nearby autoionizing states. 18 As with the NO (X 1 ) state, this experiment shows a generally increasing trend for the maximum J value and intensities of higher rotational J branches as v for NO is increased. This observation is consistent with the conclusion that the inelastic cross section for collisions between the outgoing electron and the nonspherical ion core increases as the bond distance for NO (a 3 ) is increased. ACKNOWLEDGMENTS This work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Chemical Science Division of the U.S. Department of Energy Under Contract No. W-7405-Eng-82 for the Ames Laboratory and Contract No. DE-AC03-76SF00098 for the Lawrence Berkeley National Laboratory. 1 C.-W. Hsu, M. Evans, P. A. Heinmann, and C. Y. Ng, Rev. Sci. Instrum. 68, C.-W. Hsu, P. A. Heinmann, M. 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