- Email: [email protected]
High-resolution threshold photoelectron spectrum of molecular oxygen
Chemical Phystcs North-Holland
173 (1993)
479-489
High-resolution threshold photoelectron spectrum of molecular oxygen F. Merkt ‘, P.M. Guyon LCAM Bbtunent 35 I, Umversltk Parrs XI, 91405 Orsay, France and LURE Bbment 209d, Umversib! Paris Xi, 91405 Orsay, France
and J. Hepburn Department of Chemrstry, Umversrty of Ontarro, Waterloo, Canada Received
4 January
1993
The threshold TPE photoelectron spectrum of molecular oxygen has been reinvestigated using a partially cooled effustve molecular jet and the monochromatised VUV synchrotron radiation from super-AC0 in Orsay. Three vibrational progressions are tdentttied. One of them corresponds to and confirms that observed recently by Baltzer et al. (Phys. Rev. A 45 ( 1992) 4374), the two other ones are observed for the first ttme m TPE spectroscopy. Possible assignments for these progressions are discussed. The vibrational progression in the X ‘Us state has been seen up to u+ ~26 and the two spin-orbit components (2TJ1,2 and 211s,2) are for the first ttme fully resolved in TPES. High-lying vtbratlonal levels wrth v+ > 20 of the X state are seen to overlap with the a state levels. The relative contribution of both states is obtained through a deconvolutton procedure. The vtbrational progression m the b 4x, is extended to u+ = 18. The threshold photoelectron spectrum around 20 eV shows a particularly high denstty oflines. Possible assignments of these lines to new progressions are discussed with the help of a series of time-of-flight photoelectron spectra (TOF-PES) measured at a series of excitation energies between 18 and 20 eV.
1. Introduction The purpose of this article is to present a high-resolution (40 cm- ’ ) threshold (TPE) photoelectron spectrum of oxygen measured between 12 and 22 eV at the synchrotron storage ring super-AC0 in Orsay. In the past 25 years, there have been numerous studies of O2 by conventional photoelectron spectroscopy (see for example refs. [ l-51 ) and time-of-flight photoelectron (TOF-PES) studies using tunable synchrotron radiation [ 6,7]. In addition, several lowresolution threshold photoelectron spectroscopic studies have been reported [8-l 0] and yet another report requires some justifications. The low-energy electron lines are usually not reported in conventional PES since they are embedded in a large back’ Present address: Physical of Oxford, OX 1 3QZ, UK. O301-0104/93/$06.00
Chemistry
Laboratory,
Universtty
0 1993 Elsevier Science Publishers
ground of low-energy scattered electrons. The actual origin of these electrons is not yet understood and our TPES studies show that at least some of them result from photoionisation of the gas under investigation. Indeed the low-energy electrons can well be separated by their time of flight according to their velocity and are detected with great efficiency, an ideal situation for TOF-PES studies. This type of spectroscopy, which implies a pulsed light source, has been developed around pulsed synchrotron radiation sources. The power of the method is illustrated in a recent study of autoionisation in O2 [ 71 in which a resolution of 5 meV is obtained for low-energy electrons, i.e. 25 to 50 meV. Threshold photoelectron spectroscopy (TPES) differs from PES in that the energy of the detected electrons is kept equal to zero while scanning the exciting photon energy. This is made possible by the use of tunable VUV light sources. As the frequency is
B.V. All rights reserved.
480
F. Merkt et al. / Chemcal Physuzs I73 (1993) 479-489
scanned, peaks in the yield of photoelectrons are observed at a series of photoionisation thresholds corresponding to energy differences between the neutral ground state levels and the ionic levels. While the intensities of lines observed by conventional He(I) and He( II) photoelectron spectroscopy are mostly governed by Franck-Condon factors, those observed by TPES are often dominated by autoionisation [ 1 l131. This is primarily due to the high density of Rydberg states converging on higher rovibronic ionic levels that can autoionise near a given threshold and produce the electrons of low kinetic energy detected in TPES. Another source of threshold photoelectrons, which has been observed, for example, in N20 and ought to be general in polyatomic molecules, is that of resonant autoionisation [ 14-161. Over the years, TPES has established itself as an important tool for the study of photoionisation and can be used to extract information on energy redistribution in highly excited molecular states. Autoionisation processes lead, in many cases, to the observation of ionic states that cannot be observed by conventional PES. Experiments in which ions are extracted in coincidence with threshold photoelectrons (TPEPICO experiments) are used to prepare ions in a wide range of vibronic states and to study the reactivity of state selected excited ions. A detailed understanding of the TPE spectra is essential for the design of such experiments and for the interpretation of the results. Despite considerable experimental efforts, several problems in the photoelectron spectroscopy of O2 remain unsolved: although the existence (and the weakness) of the 2 *II” state has been predicted more than 20 years ago by Dixon et al. [ 221, this state has not yet been conclusively identified. Baltzer et al. [ 51 recently reported a new, weak vibrational progression starting at 20.35 eV in the He(I) photoelectron spectrum. In the same report, an ab initio calculation of the state predicted a vibrational constant o, rather similar to the experimental value but the adiabatic binding energy was found to be some 0.66 eV lower than found experimentally. In the same region, but by He(I) and He(I1) PES from the excited ‘4 state, Jonathan et al. [ 23 ] could identify two new states at 18.66 and 19.79 eV above the neutral ground state, which they assigned to the ‘Q, and ‘4 states of the ion, respectively. It is not altogether clear whether the
new progression seen by Baltzer et al. [ 51 corresponds to higher vibrational levels of the ‘4 state or to the 2 ‘II” state, or, alternatively, whether the state seen by Jonathan et al. [ 231 is effectively the “$ state. The interest of the TPE spectrum in the region just below 20 eV is that it provides some additional information on this problem. Another purpose of this work is to provide a highresolution TPE spectrum of vibronically excited states of O2 for comparison with forthcoming ZEKE spectra [ 201. TPES and ZEKE-PES [ 17 ] have several commons features: both use angular and time discrimination to detect selectively electrons of low or zero kinetic energy as a function of the exciting wavelength. The main difference between the two techniques resides in the delayed application of the extraction field, typically 1 us, in ZEKE-PES as opposed to continuous extraction in TPES. One may thus consider that most of the zero-energy electrons come from the ionisation continua in TPES and from field ionisation of long-lived Rydberg states in ZEKE spectroscopy. The question arises whether, besides the higher resolution of the ZEKE experiments, i.e. 1 cm-’ versus 40 cm-’ due to the use of laser sources, both techniques will give the same spectroscopic information and in particular whether the vibronically excited states of the ions observed by TPES will also be seen in ZEKE. Since autoionisation is seen to play a major role in ZEKE-PES [ 18,191 as well as in TPES one may wonder whether excited neutral (Rydberg, valence) states, which autoionise to produce vibronically excited states of the ions [ 15 1, will survive long enough to be observed in the ZEKE spectra as well. The ZEKE-PES studies conducted in Waterloo on the high vibrational members of the X 211gstate of 0:) which dominate the TPES spectrum up to 16 eV, give, at least in this case, an afftrmative answer [ 20 1. The narrow link between ZEKE-PES and TPES will be discussed in more detail in a forthcoming paper
[211.
2. Experimental The data have been obtained using a double electron-ion time-of-flight (TOF) spectrometer associated with a 3 m normal incidence monochromator to disperse the VUV light from super-ACO, the new
F. Merkt et al. / Chemuzal Physics I73 (I 993) 4 79-489
Orsay synchrotron radiation positron storage ring. The double TOF spectrometer has been described in previous publications [ 241 and its characteristics are only briefly summarised here. It is mounted in a vertical position, perpendicular to the main polarisation axis of the synchrotron light. This position was chosen to minimise the width of the light beam at the intersection with the molecular jet (this width amounts to 1.8 mm (fwhm) in the horizontal direction and to only 0.2 mm in the vertical direction) and therefore achieve a higher resolution in the TOF measurements. The gas inlet, which was provided by a stainless steel needle in previous experiments, was replaced by a pinhole of 20 pm diameter through which pure oxygen (stagnation pressure 2 bar) was introduced into the ionisation region in a supersonic expansion. The molecular sample therefore showed some degree of cooling. A detailed inspection of the line profiles reveals a dominant contribution of a cold sample superimposed on a weaker room temperature spectrum. This room temperature contribution leads to the weak but wide wings observed on the low and high frequency side of all the vibronic bands (see for instance fig. 3). A quantitative analysis of the temperature is unfortunately not possible as part of these wings can also be attributed to rotational branches involving large changes of the core rotational angular momentum [ 25 1. The monochromator, equipped with a 1200 lines/mm holographic grating and 50 urn slits provided a photon bandwidth of 0.02 nm. The data were collected while the storage ring was operated in a two bunch mode with a 120 ns interval between successive light pulses. Electrons and ions produced at the center of the photoionisation region were extracted in opposite directions by a 1 V cm-’ electric field. The electrons were further accelerated and slightly focused onto the entrance aperture of the detector by a series of five electrodes maintained at low potentials between 1 and 2 V. In these conditions zeroenergy electrons reach the detector in about 100 ns, a time shorter than the super-AC0 pulse period. Energetic electrons ejected in the forward direction arrive at the detector in a shorter time while those ejected in the opposite direction are either lost or are stopped by the field, return, and arrive later. The high negative voltage on the ion flight tube of the TOF spectrometer was turned off during these measurements as it resulted in a large background
481
signal in the TPE spectra and in spurious signals in the TOF spectra. These spurious electron signals have two origins: first the fast electrons of energy > 0.5 eV that are ejected along the detection axis but away from the electron detector are reflected by the high negative voltage of the ion flight tube. Secondly, metastable molecules produced in high Rydberg states and which are not field ionised by the extraction field of 1 V cm-’ may survive until they enter the region around the ion flight tube, where they are field ionised by the strong electric field ( > 300 V cm-‘) between the ion extraction electrode and the flight tube entrance grid electrode. These electrons are subject to a high potential and are reflected back towards the electron detector. The TOF spectra were obtained using the electron signal as a start pulse for a Canberra time to amplitude converter and the pick up signal detected in the storage ring at the passage of the positron bunches as a stop. The analog pulses were analysed and stored in a Lecroy QVT multichannel analyser that was interfaced with a Macintosh II computer by way of a Camac GPIB interface and a GPIB-Macintosh National instrument board. The threshold photoelectron spectrum of argon recorded at the same conditions as the spectra discussed in this paper is presented in fig. 1. The ionisation limits of Ar (2P,,2 and 2P3,2) can be seen as intense peaks. The transmission function for the threshold electrons can be derived immediately from the profiie of the 2P,,2 peak. A background of 0.5Oh
I
I
Ar TPES I
I
I
15.9
15.95
h 2P3,215.7597
1’5.7
15.75
15.x
15.85 Encrgy(cV)
Fig. 1. TPE spectrum of Ar. Test of the resolution.
16
482
F. Merkt et al. /Chemical Physics 173 (I 993) 4 79-489
of the total ionisation can be seen at all frequencies. If it is subtracted from the TPE spectrum, one sees that electrons of 30 meV kinetic energies are still transmitted with 5Oh efficiency. This explains the presence of a weak line on the high energy side of the 2P,,2 limit that corresponds to electrons produced by spin-orbit autoionisation on the near-lying Rydberg state belonging to the series converging to the *P,,* limit. As a result, the lines whose intensity is smaller than 5% of the most intense peak in the TPE spectrum cannot be unambiguously assigned to ionic states, but might originate from autoionisation of a Rydberg state lying up to 30 meV above an ionic state. The dynamical range of intensities that can be probed by TPES therefore amounts to approximately 20. Whether weak lines can effectively be attributed to an ionic state must be checked by time-of-flight photoelectron spectroscopy as is illustrated in this work. The energy calibration of the TOF-PES was performed by measuring the TOF of electrons released from Ar as a function of the exciting wavelength. The TOF spectra of the first vibrational members of the 0: b state were also used in the calibration. The accuracy of the calibration is expected to be of the order of 10 meV for the lines that correspond to electrons emitted with less than 1 eV kinetic energy. The intensity of the peaks measured in the TOF spectra was determined by weighting the experimental data with the transmission function of the TOF spectrometer, which was determined from TOF photoelectron spectra of Ar. The intensity of the peaks is expected 0,’
to become more accurate for signals corresponding electrons emitted with increasing kinetic energy.
3. Results and discussion For the analysis, particular attention is given to four regions of the spectrum for which the results can be presented as follows. 3.1. The X ‘II, state of 0: in the region of the a “II,, state(15.4-17eV) A particularly striking feature of the TPE spectrum of 02 near the onset of the a 411Ustate is the long progression due to high vibrational members of the X 2Hs state. As can be seen from fig. 2, the two spin-orbit components of the X state are well resolved and the progression can be followed up to v+ = 26. Moreover, the intensity of the high vibrational components is strong enough to obscure the onset of the a state in the TPE spectrum. This is in sharp contrast to the He (I) [ 1 ] and He (II) [ 5 ] photoelectron spectra in which the vibrational progression in the X state stops approximately at U+ = 5, a consequence of vanishing Franck-Condon factors. The Ne( I) PE spectrum however is rather similar to the TPE spectrum, in that the X state is readily observed up to v+ > 20 [ 41. The non-Franck-Condon behaviour observed in the TPE spectrum can be attributed to the high density, in this spectral region, of autoionising Rydberg states that belong to series converging to the a and A states. Au-
TPES
X
Energy Fig. 2. TPES of Q between 15.4 and 17 eV. The spectrum overlaps the first vibrational members of the a state.
to
and
a states
(eV)
shows a very long vibrational
progression
in the X state, which partially
I? Merkt et al. /Chemrcal Physics 173 (1993) 475489
toionisation of these states to the high vibrational levels of the X state produces the electrons of low kinetic energy that constitute the TPE signal. The overlap between the high V+ members of the X state and low U+ members of the a state could, in principle, have some bearing on the interpretation of reaction dynamical experiments that rely on the TPEPICO technique to prepare 0: ions in the low vibrational members of the a state [ 26,271. For this reason, we have attempted to determine the percentA
JOO _,
300
_
200
_
loo_
O_
$
16.05
16.10
16.15
z
W
16.20
16.30
16.25
16.35
16.30
16140
483
age of the TPES signal that stems from the a and X states in the regions surrounding the v+ =O to v+ = 3 components of the a state. Details of the TPE spectrum in these regions can be seen in figs. 3a-3c. The percentages of X and a state obtained by fitting each profile with three Gaussians are summarised in table 1. The width assumed for the members of the X and a state was taken to be that measured from non-overlapping X and a lines in the TPE spectrum (40 and 80 cm-’ respectively) and that for the a state is the same width as measured for the v+ = 3,4 and 5 peaks which are separated from the X state transitions but represent an unresolved contribution of four spinorbit sublevels. The “deconvolution” procedure does not take into account the weak but wide wings that are observed on both sides of the bands and which are due to either a room-temperature contribution to the spectrum or to rotational branches associated with large core angular momentum changes. This is however not significant for the present analysis as these wings amount to less than 10% of the overall peak intensities. The X and a contribution presented in table 1 are supported by the observations that the intensities of those low members of the a state that are not overlapped by the X state correspond well to the intensities in the He(I) PE spectrum [ 5 ] and to calculated Franck-Condon factors and that, after deconvolution, the intensity of the u+ =O-3 components of the a state also corresponds to FranckCondon factors. The TPES intensities of the first vibrational members of the a state are shown in table 2. The values indicated in table 2 are normalised to the value of the v + = 4 state that is arbitrarily given a value of 100. The contrast between the regions above and below the a state is striking since Rydberg levels converging to the a and A state are seen to autoionise to the X Table 1 Percentage of the TPES stgnal which stems from the X and a states of 0: in the vicmity of the lowest four vibrational members of the a state
ENERGY/eV
Fig. 3. Details of the regions of the TPE spectrum of O2 in which high v+ members of the X state of 0: overlap the first member of the a state. (a) TPE spectrum in the region of the a (u+ ~0) and X (v+ = 2 1) states. (b ) TPE spectrum in the region of the a (v+ = 1) and X (v+ = 22) states. (c) TPE spectrum in the region ofthea (v+=2) andX (v+=23) states.
X(v+=2l),a(v+=O) X(v+=22),a(v+=l) X(v+=23),a(v+=2) X(v+=24),a(v+=3)
X state
a state
70% 55% 35% 15%
30% 45% 65% 85%
484
F. Merkt et al. /Chemical
Physics I:73 (I 993) 4 79-489
Table 2 Line intensities derived from the TPE spectrum of the a state of 0:. Aurelio Ferreira V+
0
1 2 3 4
Comparison with Franck-Condon factors from the thesis of L.F.
hw (eV)
hv (eV)
this work
calculated
Intensity (counts/s)
Intensity normalised
FCF normalised
16.102 16.227 16.350 16.471 16.590
16.101 16.221 16.350 16.470 16.589
115 430 850 I200 1430
8.0 36.1 59.4 83.9 100
7.9 28.9 51.9 84.2 100
state of 0: whereas Rydberg levels converging to the b state and situated above the a (v+ = 0) limit do not seem to do so. Inspection of the high-resolution photoionisation yield spectrum of Dehmer and Chupka in this region, i.e. 740-770 A, shows that the Rydberg levels are weak and diffuse [ 29 1. They are likely to be predissociated since Carlson observed a strong emission from excited neutral oxygen atoms in this region of the spectrum [ 28 1. 3.2. The b *t;; state of 0:
between 18 and 20 eV
An interesting question in this region is whether autoionisation leads to the observation of high vibrational levels of the b state as is true for the X state. The TPE spectrum of the b state is presented in fig. 4. The intensities of the first vibrational members up to U+ = 7 fall off quicker than predicted from FranckCondon factors as can be seen from table 3 (columns 4 and 5). The photoionisation spectrum in this re-
r
gion is dominated by several series of autoionising Rydberg states that converge to the first three vibrational members of the B state [ 291. These states autoionise predominantly to the b state [ 301, and the comparable geometry of the b and B states imply predominantly autoionisation with A (v+ =O). It is therefore not surprising that the low vibrational levels of the b state appear too intense compared to that of higher vibrational levels. The vibrational progression in the b state can be followed up to v+ = 18 even though the intensities of the members with v+ > 8 are very weak and appear rather irregular. The line positions measured in the TPE spectrum are compared in table 3 (column 2 and 3) with the values predicted using a Morse potential with w, = 1196.77 cm-’ and o&x, = 17.09 cm-’ from ref. [ 3 1 ] and an adiabatic ionisation potential of 18.17 1 eV [ 5 1. A second-degree polynomial fit of the data yields o,= 119 1.3 cm-’ and oar= 17.14 cm-’ which is in very close agreement. We calculate a dissociation energy of 2.576
02+ b 4zg---Y--------1-
-------
-~- - _I_r_---
1
I
“18
18.5
19.5 19 Energy (eV)
20.5
Fig. 4. TPE spectrum of the 0: b *Z; +-Or X ‘Z; transition. The vibrational progression in the b state can be followed up to v+ = 18.
F. Merkt et al. / Chemical Physics 173 (I 993) 4 79-489
485
Table 3 Line positions of the transitions to the different vibrational members of the b state. The values are compared with values calculated using a Morse potential II+
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
hv (eV) exp. 18.171 18.314 18.452 18.586 18.716 18.847 18.970 19.085 19.200 19.308 19.416 ( 19.626) 19.793 19.878 19.955 20.036 20.098
hv (eV) talc.
I (counts/s)
I normalised
FCF normalised
18.717 18.315 18.455 18.591 18.722 18.849 18.972 19.091 19.205 19.316 19.422 19.524 19.621 19.714 19.803 19.886 19.968 20.039 20.117
3700 2400 980 200 80 93 55 35 15 43 25
100 64.9 26.5 5.4 2.1 2.5 1.5
100 81.9 39.6 15.1 5.0 1.6 0.5
eV and a dissociation limit of 20.747 eV very close to the thermodynamic value of 20.7 eV. The observation in the TPE spectrum of transitions to these high vibrational members of the b state in the FranckCondon gap between the b and B states is somewhat surprising given the comment made above that autoionisation from the Rydberg states converging to the B state should be governed by a Au+ = 0 propensity rule [ 321, and should therefore not lend intensity to TPES lines corresponding to v+ > 8. A possible explanation for the presence of these lines could be that their intensity is gained from autoionisation from an as yet unidentified Rydberg series converging to the ionic state observed recently by Baltzer et al. [ 51 around 20.35 eV (see also sections 3.3 and 3.4 below). Alternatively, these electrons could be produced in a process similar to that of resonant autoionisation. In fact, the dissociation limit of 0; lies at 18.733 eV. Above this limit, the b and B states are known to be predissociated [ 331 so that the autoionisation of the Rydberg electron could occur while the molecular ion core dissociates, leading to a large change of geometry. Alternatively one may consider the autoionisation of repulsive Rydberg states with
low principal quantum number converging to the 3 *II” state whose vertical IP is observed at 23 eV [ 5,9 1. Evidence for such processes involving dissociative neutral states is given by the observation of 02+ B’C
19.6
20
P
20.4 20.8 Energy(eV)
21.2
21.6
Fig. 5. TPE spectrum of the 0: B *Z; to2 X ‘2; transition. The lower frequency part of the spectrum corresponds to the high frequency part of fig. 4. Note the presence of a weak additional progression in this region.
486
F Merkt et al. / Chemrcal Phystcs 173 (I 993) 4 79-489
0* autoionising lines in the PES of Cafolla et al. [ 341. Such atomic transitions result from autoionisation of excited oxygen atoms produced by the dissociation of the excited molecule. They are also seen in the present study, see section 3.4. Two other weaker progressions are also seen above 19.6 eV, they are identified with the help of the TOF-PE spectra and discussed in section 3.4. 3.3. The TPES in the region of the B ‘Z; 0: (20 and 21.5 ev)
0
1
2
3
I
5
/
b - STATE
6
state of
This region of the spectrum deserves special attention since Baltzer et al. [ 5 ] have recently and for the first time identified a new progression in the photoelectron spectrum of molecular oxygen. However, some doubt persists concerning the assignment of this progression. Two candidates have been proposed by Baltzer et al. [ 51: the 2Ag, which was first observed by Jonathan et al. [ 231 in the He(I) PES spectrum of 02, excited the ‘Ag state and the 2 *II” state which still remains to be identified. In ref. [ 5 1, the experiment was compared with an ab initio calculation of the potential energy surface of the 2 211u which predicted a vibrational spacing rather similar to that measured experimentally. However, the experimental binding energy was found to be some 0.66 eV higher than the theoretical prediction. A possible explanation for this discrepancy could be that the first vibrational members of the progression cannot be seen in the PE spectrum. If this is the case, they might be observable in the TPE spectrum, if they gain intensity by autoionisation. The threshold photoelectron spectrum of 0, in the region of the B state is displayed in fig. 6. Except for the v+ = 4 level which appears unexpectedly weak, the vibrational progression of the B state does not show significant intensity perturbations compared to that observed in the high-resolution He (I ) PES. In addition, all the lines observed by Baltzer et al. [ 5 1, and attributed to the new progression, can be seen in the TPES, with comparable intensities. The effect of autoionisation on the TPE spectrum of the X state between 15 and 17 eV differs considerably from that observed in the region of the B state between 20 and 2 1.5 eV. In the former case, the autoionisation lines are sharp and lead to non-Franck-Condon intensity distributions (see section 3.1). In the latter case, the
Fig. 6. Time-of-flight photoelectron spectra of 02, measured at different excitation frequencies between 18 and 20 eV: (a) 18.850 eV; (b) 19.090 eV; (c) 19.805 eV; (d) 19.945 eV; (e) 19.965 eV.
autoionisation resonances are particularly broad [ 29 ] and, while undoubtedly contributing to the threshold ionisation, do not seem to perturb the TPES intensities as drastically. Table 4 summarises the observed line positions and compares them with the values reported by Baltzer et al. [ 51. A second-degree polynomial fit of the data yields c&=805 cm-‘, 0~~ = 12.7 cm- ’ from which a dissociation energy of 1.58 eV is calculated with a Morse potential function. This yields a dissociation limit for this state of 2 1.93 eV close to the third thermodynamic limit 22.06 eV. One of the reasons to investigate the TPE spectrum in that region was to try and see whether the origin of the progression was effective at 20.35 eV and not at a lower frequency as suggested by the theoretical pre-
F. Merkt et al. /Chemical Physics 173 (1993) 4 79-489
Table 4 Line positions of the transitions to the different vibrational members of the weak progression observed in the vicinity of the B state. The values are compared with those obtained in the He(I) PE spectrum of Baltzer et al. [ 51. The assignment corresponds to that proposed by Baltzer et al. V+
0 1 2 3 4 5 6 7
hv (eV) this work
hv (eV)
20.350 20.448 20.545 20.636 20.724
20.35 1 20.450 20.544 20.637 20.726 20.810 20.890 20.968
20.890 20.973
Baltzer et al. [ 51
Indeed, it is well known that TPES signals can be detected in situations where no intensity is measurable by conventional PES. Unfortunately, the density of lines in the TPE spectrum between 19.6 and 20.3 is so high that it is not difficult to find a series of lines which would (accidentally or not ) fit with the rest of the progression. It is therefore justitied to ask whether some of them are due to autoionisation processes which release electrons of low kinetic energy. To clarify the situation, several TOFPES have been measured for a series of excitation frequencies between 19.8 and 20 eV. diction.
3.4. TOFphotoelectron spectra in the region just below 20 eV(19.6-20 ev) The high density of lines in the TPES just below 20 eV was noted at the end of the previous subsection. Some of these lines correspond to the high vibrational members of the b state and some of them might be assigned to the first members of the weak vibrational progression which partially overlaps the B state (see section 3.3). A series of TOF photoelectron spectra covering that region has been measured at several different excitation frequencies between 18 and 20 eV. The results are displayed in fig. 6. The TOF spectra a and b, which show the first vibrational levels of the b state, were used to calibrate the TOF spectrometer in energy. The spectra c-e were obtained by ionising the sample with photons of 19.805, 19.945 and 19.965 eV energy, respectively. In all these
487
TOF spectra, the progression in the b state can be unambiguously followed up to u+ = 7. Several lines which stem from autoionisation of excited 0 atoms produced by dissociation are indicated with dotted lines in fig. 6. These lines can be identified relatively easily in the TOF spectra as the kinetic energy of the photoelectrons does not depend on the excitation wavelength but solely on the energy difference between the metastable neutral atomic state and O+ ionic levels. The electrons emitted by atomic autoionisation have therefore the same time of flight in each spectrum and, as a result, appear at different energies after the TOF scale has been transformed to an energy scale. The occurrence of such atomic lines in the PES spectrum of O2 have been recognised previously by Cafolla et al. [ 341. A total of six lines are resolved in the TOF spectra between 19.6 and 20 eV and can be classified in two groups. Four intense peaks are almost equidistant and seem to belong to the same progression, their positions are reported in table 5. The spacing between these successive peaks (approximately 100 meV) does not correspond to the spacing of 114 meV, which would be expected if these lines were members of the progression discussed in the previous subsection. An assignment of these peaks to the first levels of the corresponding ionic state can therefore be ruled out. This spacing of 100 meV corresponds well to the spacing one would expect between two successive members of the b state in that region. However, the energies of the peaks appear to be displaced by approximately 20 meV from the position of the levels of the b state. In addition, if these peaks were to be assigned to vibrational members of the b state with u+ = 12-15, one
Table 5 Line positions of the transitions to the different vibrational members of the new weak progression observed below the B state lJ+
hv (eV) this work
0 1 2 3
19.643 19.743 19.836 19.924
0 1
19.78 19.89
hv (eV) Jonathan [ 231
19.79 19.90
488
F. Merkt et al. /Chemical Physm 173 (I 993) 479-489
ought to see z,+ = 11 as well in the TOF-PE spectra, which is not the case as can be seen from figs. 6c and 6e. These states seem therefore to form an additional progression starting at 19.64 eV with a vibrational spacing of approximately 100 meV. This is the first time this progression has been observed in a photoelectron spectrum. The reason for its occurrence in the TOF spectra presented here might be that it gains intensity from autoionisation. Two additional lines can be seen in the TOF spectra. Their intensity seems to depend strongly on the excitation frequency: the peak at 19.78 eV is barely noticeable in the TOF spectrum (e) but is the most intense in the TOF spectrum (c). Their frequencies ( 19.78 and 19.89 eV) correspond well to the frequencies of the first two members of the ‘4 state ( 19.79, 19.90) seen by Jonathan et al. [ 231. It cannot be ruled out at this stage that these two lines belong to the same progression as the lines observed by Baltzer [ 51 which overlap the B state (see section 3.3). We may now propose an assignment of the three weak transitions, reported in tables 4 and 5, based on the work of Beebe et al. [ 35 1. These authors carried out an ab initio calculation of all the bound states of 0:. The calculated ionisation potentials are within 0.2 eV of the experimental values for the known states. The two lines observed at 19.78 and 19.89 eV belong to the ‘A, state as discussed above. For the remaining four lines seen below 20 eV in the TOF-PES and which form a progression starting at 19.643 eV, we estimate a dissociation limit of 20.9 eV, close to the second dissociation limit of 0: at 20.7 eV. The “4 state with a calculated IP of 20.07 eV and o,= 82 meV seems an attractive candidate. Finally we find two candidates for Baltzer’s progression starting at 20.35 eV and having a vibrational constant of 100 meV: the 2’l-I” with an IP of 19.55 eV and w,= 120 meV as already proposed by Baltzer et al. [ 5 ] or the 2 ‘IIs state with an IP of 20.45 eV and w,= 120 meV. Since both states dissociate to the same limit at 22.06 eV the latter assignment may be considered as preferable but a definite conclusion will await further investigations.
4. Conclusions The high-resolution TPE-spectrum of partially cooled O2 has been measured between 12 and 22 eV and has revealed many new features of the 0: spectrum, some of them are very weak but nevertheless definitely identified as ionic levels. The two spin-orbit components of the X state are fully resolved and the vibrational progression in the X state can be followed up to v+ ~27. The intensity of the high v+ members of the X state is strong enough to obscure the onset of the a state. A deconvolution of the TPES spectrum in a and X contributions is performed in order to assist in the interpretation of reaction dynamical experiments which rely on the TPEPICO method to prepare 0: ions in the first levels of the a state. The vibrational progression in the b state of 0: is visible up to v+ = 18. The source of intensity of the vibrational components with v+ > 8 is not clear as the photoionisation spectrum in that region is dominated by autoionisation of Rydberg states belonging to series converging on the first vibrational levels of the B state [ 29,301 and that, due to the similar geometries of the B and b states, autoionisation from the B state to the b state should be governed by Au+ = 0 processes and should therefore not favour the production of high vibrational members of the b state. The intensity patterns observed for the B state are similar to those obtained by He(I) PES, indicating that autoionisation is not a dominant source of intensity in that region. An additional weak and yet unassigned vibrational progression which was first observed by Baltzer et al. [ 5 ] is clearly seen in the TPE spectrum. The present analysis confirms the results obtained in ref. [ 5 ] but we propose a different assignment. It is possible that this progression starts at a lower frequency than found by Baltzer et al. but the high density of the lines in the threshold photoelectron spectrum and the weakness of the lines which could be identified as the first members of this new progression prevent a definitive assignment. Further work on the isotopic ‘*O2 molecule is in progress to solve the problem. TOF photoelectron spectra have been measured in the energy region below 20 eV in order to confirm the assignment of some of the weak TPES lines. These spectra reveal a series of six lines between 19.6 and 20 eV. Four of these, with energies 19.643, 19.743,
F. Merkt et al. /Chemcal
19.836 and 19.924 seem to form an as yet unobserved progression. The intensities of the remaining two lines are very sensitive to the excitation wavelength and their position corresponds well to the energy of the first two vibrational levels of the *Agstate of 0: seen by Jonathan et al. [ 23 ] which is observed for the first time in the PES of normal Oz. Additional work on isotopes of O2 is currently in progress to provide definitive answers to the remaining questions. The use of high-resolution time-of-flight photoelectron spectroscopy (TOF-PES) in combination with threshold photoelectron spectroscopy (TPES) has been shown to be efficient in extending the dynamic range of the TPES method to low intensity lines, i.e. less than 2.5% of the main lines, which could not otherwise be unambiguously identified with ionic levels because of autoionization.
Physics 173 (1993) 479-489
489
[7] P.M. Guyon, J. Hepburn, T. Weng, F. Heiser Reynolds, Phys. Rev. Letters 67 (1991) 675. 181 R. Stockbauer,
Advan. Mass. Spectrom.
[9] P.M. Guyon and I. Nenner,
8 (1980)
Appl. Opt. 19
[ 101 J.M. Ajello and A. Chutjian,
and 79.
( 1980) 4068.
J. Chem. Phys. 71 (1979)
] 111 P.M. Guyon, R. Spohr, W.A. Chupka Chem. Phys. 65 ( 1976) 1650.
D.
1079.
and J. Berkowitz,
J.
[ 121 W.B. Peatman, J. Chem. Phys. 64 (1976) 4368. [ 131 T. Baer and P.M. Guyon, J. Chem. Phys. 85 (1986)
4765.
[ 141 T. Baer, P.M. Guyon, I. Nenner, A. Tabche-Fouhailt, Botter, L.F.A. Ferreira (1979) 1585. ]15
P.M. Guyon, (1983)
R.
and T.R. Govers, J. Chem. Phys. 70
T. Baer and I. Nenner,
J. Chem.
Phys.
78
3665.
116 W.A. Chupka and E. Eyler, J. Chem. Phys. 88 ( 1988) 3032. M. Sander and E.W. Schlag, Chem. ]17 K. Miiller-Dethlefs, Phys. Letters
112 (1984)
291.
[ 181 F. Merkt and T.P. Softley, J. Chem. Phys. 96 (1992)
4149.
[ 19 ] F. Merkt and T.P. Softley, Phys. Rev. A 46 ( 1992) 302. [20] J. Hepburn
et al., to be published.
I 1F. Merkt and P.M. Guyon, to be published. 122] R.N. Dixon and S.E. Hull, Chem. Phys. Letters 3 ( 1969) [2
Acknowledgement We thank the staff of LURE for the financial and technical support of this experiment and for operating the super-AC0 storage ring. We thank F. Heiser and 9. Alonsi for helping us with some of the data handling. We are especially grateful to 9. Lagarde and P. Martin who were in charge of the 3 m monochromator installation on the SA 63 beam line and we thank the Ministhe des Affaires Etrangeres, France, for financial support to one of us (FM).
367. A. Morris, M. Okuda, K.J. Ross and D.J. ]23 ] N. Jonathan, Smtth, J. Chem. Sot. Faraday Trans. II 70 (1974) 1810. ]24 ] M.R. Viard, 0. Dutuit, 0. Atabek and P.M. Guyon, J. Chem. Phys. 93 (1990) 8881; I. Powis, 0. Dutmt, M. Richard-Viard and P.M. Guyon, J. Chem. Phys. 92 ( 1990) 1643. [25] R. Tonkm, Letters
J.W. Winmczek
164 (1989)
and M.G. White, Chem. Phys.
137.
[26] K. Tanaka, T. Kato, P.M. Guyon Phys. 92 (1982) 1643.
J. Chem. Phys. 60 (1974)
[29] P.M. Dehmer and W.A. Chupka,
[ 1] 0. Edqvist, E. Lindholm, L.E. Selin and L. Asbrink, Physica Scripta 1 (1970) 25. [2] J.L. Gardner and J.A.R. Samson, J. Chem. Phys. 62 (1975) 4460. [ 31 H. van Lonkhuyzen and C.A. de Lange, J. Electron Spectry. 27 (1982) 255. [4] J.A.R. Samson and J.L. Gardner, J. Chem. Phys. 67 (1977) 755. [ 51 P. Baltzer, B. Wannberg, L. Karlsson, M. Carlsson-Gothe and M. Larsson, Phys. Rev. A 45 (1992) 4374. [6] P. Morin, I. Nenner, P.M. Guyon, 0. Dututt and K. Ito, J. Chim. Phys. 77 ( 1980) 78; P. Morin, I. Nenner, P.M. Guyon, L.F.A. Ferretra and K. Ito, Chem. Phys. Letters 75 ( 198 1) 1129.
J. Chem.
[ 27 ] C. Alcaraz, 0. Dutuit and P.M. Guyon, to be published. [28] R.W. Carlson,
References
and I. Koyano,
2350.
J. Chem. Phys. 62 (1975)
4525. [30] A. Tabche-Fouhatlt, I. Nenner, P.M. Guyon Delwiche, J. Chem. Phys. 75 (1981) 1129.
[ 311 K.P. Huber molecular molecules
and
G. Herzberg,
spectra
structure, Vol. IV. Constants of (Van Nostrand, New York, 1979).
[32] P. Morin, I. Nenner, Delwiche,
Molecular
and
and
diatomtc
M.Y. Adam, M.J. Hubin-Franskin,
H. Levebvre-Brian
Phys. Letters 92 (1982)
and A. Giusti-Suzor,
J.
J.
Chem.
609.
[ 33 ] P.M. Guyon, T. Baer, L.F.A. Ferreira, I. Nenner, A. TabcheFouhaile,
R. Botter and T. Govers,
LI, and references
J. Phys. B 11 (1978)
therein.
[ 341 A.A. Cafolla, T. Reddish and J. Comer, J. Phys. B 22 ( 1989) L273.
[ 35 ] N.H.F. Beebe, E.W. Thulstrup Phys. 64 (1976)
2080.
and A. Andersen,
J. Chem.
173 (1993)
479-489
High-resolution threshold photoelectron spectrum of molecular oxygen F. Merkt ‘, P.M. Guyon LCAM Bbtunent 35 I, Umversltk Parrs XI, 91405 Orsay, France and LURE Bbment 209d, Umversib! Paris Xi, 91405 Orsay, France
and J. Hepburn Department of Chemrstry, Umversrty of Ontarro, Waterloo, Canada Received
4 January
1993
The threshold TPE photoelectron spectrum of molecular oxygen has been reinvestigated using a partially cooled effustve molecular jet and the monochromatised VUV synchrotron radiation from super-AC0 in Orsay. Three vibrational progressions are tdentttied. One of them corresponds to and confirms that observed recently by Baltzer et al. (Phys. Rev. A 45 ( 1992) 4374), the two other ones are observed for the first ttme m TPE spectroscopy. Possible assignments for these progressions are discussed. The vibrational progression in the X ‘Us state has been seen up to u+ ~26 and the two spin-orbit components (2TJ1,2 and 211s,2) are for the first ttme fully resolved in TPES. High-lying vtbratlonal levels wrth v+ > 20 of the X state are seen to overlap with the a state levels. The relative contribution of both states is obtained through a deconvolutton procedure. The vtbrational progression m the b 4x, is extended to u+ = 18. The threshold photoelectron spectrum around 20 eV shows a particularly high denstty oflines. Possible assignments of these lines to new progressions are discussed with the help of a series of time-of-flight photoelectron spectra (TOF-PES) measured at a series of excitation energies between 18 and 20 eV.
1. Introduction The purpose of this article is to present a high-resolution (40 cm- ’ ) threshold (TPE) photoelectron spectrum of oxygen measured between 12 and 22 eV at the synchrotron storage ring super-AC0 in Orsay. In the past 25 years, there have been numerous studies of O2 by conventional photoelectron spectroscopy (see for example refs. [ l-51 ) and time-of-flight photoelectron (TOF-PES) studies using tunable synchrotron radiation [ 6,7]. In addition, several lowresolution threshold photoelectron spectroscopic studies have been reported [8-l 0] and yet another report requires some justifications. The low-energy electron lines are usually not reported in conventional PES since they are embedded in a large back’ Present address: Physical of Oxford, OX 1 3QZ, UK. O301-0104/93/$06.00
Chemistry
Laboratory,
Universtty
0 1993 Elsevier Science Publishers
ground of low-energy scattered electrons. The actual origin of these electrons is not yet understood and our TPES studies show that at least some of them result from photoionisation of the gas under investigation. Indeed the low-energy electrons can well be separated by their time of flight according to their velocity and are detected with great efficiency, an ideal situation for TOF-PES studies. This type of spectroscopy, which implies a pulsed light source, has been developed around pulsed synchrotron radiation sources. The power of the method is illustrated in a recent study of autoionisation in O2 [ 71 in which a resolution of 5 meV is obtained for low-energy electrons, i.e. 25 to 50 meV. Threshold photoelectron spectroscopy (TPES) differs from PES in that the energy of the detected electrons is kept equal to zero while scanning the exciting photon energy. This is made possible by the use of tunable VUV light sources. As the frequency is
B.V. All rights reserved.
480
F. Merkt et al. / Chemcal Physuzs I73 (1993) 479-489
scanned, peaks in the yield of photoelectrons are observed at a series of photoionisation thresholds corresponding to energy differences between the neutral ground state levels and the ionic levels. While the intensities of lines observed by conventional He(I) and He( II) photoelectron spectroscopy are mostly governed by Franck-Condon factors, those observed by TPES are often dominated by autoionisation [ 1 l131. This is primarily due to the high density of Rydberg states converging on higher rovibronic ionic levels that can autoionise near a given threshold and produce the electrons of low kinetic energy detected in TPES. Another source of threshold photoelectrons, which has been observed, for example, in N20 and ought to be general in polyatomic molecules, is that of resonant autoionisation [ 14-161. Over the years, TPES has established itself as an important tool for the study of photoionisation and can be used to extract information on energy redistribution in highly excited molecular states. Autoionisation processes lead, in many cases, to the observation of ionic states that cannot be observed by conventional PES. Experiments in which ions are extracted in coincidence with threshold photoelectrons (TPEPICO experiments) are used to prepare ions in a wide range of vibronic states and to study the reactivity of state selected excited ions. A detailed understanding of the TPE spectra is essential for the design of such experiments and for the interpretation of the results. Despite considerable experimental efforts, several problems in the photoelectron spectroscopy of O2 remain unsolved: although the existence (and the weakness) of the 2 *II” state has been predicted more than 20 years ago by Dixon et al. [ 221, this state has not yet been conclusively identified. Baltzer et al. [ 51 recently reported a new, weak vibrational progression starting at 20.35 eV in the He(I) photoelectron spectrum. In the same report, an ab initio calculation of the state predicted a vibrational constant o, rather similar to the experimental value but the adiabatic binding energy was found to be some 0.66 eV lower than found experimentally. In the same region, but by He(I) and He(I1) PES from the excited ‘4 state, Jonathan et al. [ 23 ] could identify two new states at 18.66 and 19.79 eV above the neutral ground state, which they assigned to the ‘Q, and ‘4 states of the ion, respectively. It is not altogether clear whether the
new progression seen by Baltzer et al. [ 51 corresponds to higher vibrational levels of the ‘4 state or to the 2 ‘II” state, or, alternatively, whether the state seen by Jonathan et al. [ 231 is effectively the “$ state. The interest of the TPE spectrum in the region just below 20 eV is that it provides some additional information on this problem. Another purpose of this work is to provide a highresolution TPE spectrum of vibronically excited states of O2 for comparison with forthcoming ZEKE spectra [ 201. TPES and ZEKE-PES [ 17 ] have several commons features: both use angular and time discrimination to detect selectively electrons of low or zero kinetic energy as a function of the exciting wavelength. The main difference between the two techniques resides in the delayed application of the extraction field, typically 1 us, in ZEKE-PES as opposed to continuous extraction in TPES. One may thus consider that most of the zero-energy electrons come from the ionisation continua in TPES and from field ionisation of long-lived Rydberg states in ZEKE spectroscopy. The question arises whether, besides the higher resolution of the ZEKE experiments, i.e. 1 cm-’ versus 40 cm-’ due to the use of laser sources, both techniques will give the same spectroscopic information and in particular whether the vibronically excited states of the ions observed by TPES will also be seen in ZEKE. Since autoionisation is seen to play a major role in ZEKE-PES [ 18,191 as well as in TPES one may wonder whether excited neutral (Rydberg, valence) states, which autoionise to produce vibronically excited states of the ions [ 15 1, will survive long enough to be observed in the ZEKE spectra as well. The ZEKE-PES studies conducted in Waterloo on the high vibrational members of the X 211gstate of 0:) which dominate the TPES spectrum up to 16 eV, give, at least in this case, an afftrmative answer [ 20 1. The narrow link between ZEKE-PES and TPES will be discussed in more detail in a forthcoming paper
[211.
2. Experimental The data have been obtained using a double electron-ion time-of-flight (TOF) spectrometer associated with a 3 m normal incidence monochromator to disperse the VUV light from super-ACO, the new
F. Merkt et al. / Chemuzal Physics I73 (I 993) 4 79-489
Orsay synchrotron radiation positron storage ring. The double TOF spectrometer has been described in previous publications [ 241 and its characteristics are only briefly summarised here. It is mounted in a vertical position, perpendicular to the main polarisation axis of the synchrotron light. This position was chosen to minimise the width of the light beam at the intersection with the molecular jet (this width amounts to 1.8 mm (fwhm) in the horizontal direction and to only 0.2 mm in the vertical direction) and therefore achieve a higher resolution in the TOF measurements. The gas inlet, which was provided by a stainless steel needle in previous experiments, was replaced by a pinhole of 20 pm diameter through which pure oxygen (stagnation pressure 2 bar) was introduced into the ionisation region in a supersonic expansion. The molecular sample therefore showed some degree of cooling. A detailed inspection of the line profiles reveals a dominant contribution of a cold sample superimposed on a weaker room temperature spectrum. This room temperature contribution leads to the weak but wide wings observed on the low and high frequency side of all the vibronic bands (see for instance fig. 3). A quantitative analysis of the temperature is unfortunately not possible as part of these wings can also be attributed to rotational branches involving large changes of the core rotational angular momentum [ 25 1. The monochromator, equipped with a 1200 lines/mm holographic grating and 50 urn slits provided a photon bandwidth of 0.02 nm. The data were collected while the storage ring was operated in a two bunch mode with a 120 ns interval between successive light pulses. Electrons and ions produced at the center of the photoionisation region were extracted in opposite directions by a 1 V cm-’ electric field. The electrons were further accelerated and slightly focused onto the entrance aperture of the detector by a series of five electrodes maintained at low potentials between 1 and 2 V. In these conditions zeroenergy electrons reach the detector in about 100 ns, a time shorter than the super-AC0 pulse period. Energetic electrons ejected in the forward direction arrive at the detector in a shorter time while those ejected in the opposite direction are either lost or are stopped by the field, return, and arrive later. The high negative voltage on the ion flight tube of the TOF spectrometer was turned off during these measurements as it resulted in a large background
481
signal in the TPE spectra and in spurious signals in the TOF spectra. These spurious electron signals have two origins: first the fast electrons of energy > 0.5 eV that are ejected along the detection axis but away from the electron detector are reflected by the high negative voltage of the ion flight tube. Secondly, metastable molecules produced in high Rydberg states and which are not field ionised by the extraction field of 1 V cm-’ may survive until they enter the region around the ion flight tube, where they are field ionised by the strong electric field ( > 300 V cm-‘) between the ion extraction electrode and the flight tube entrance grid electrode. These electrons are subject to a high potential and are reflected back towards the electron detector. The TOF spectra were obtained using the electron signal as a start pulse for a Canberra time to amplitude converter and the pick up signal detected in the storage ring at the passage of the positron bunches as a stop. The analog pulses were analysed and stored in a Lecroy QVT multichannel analyser that was interfaced with a Macintosh II computer by way of a Camac GPIB interface and a GPIB-Macintosh National instrument board. The threshold photoelectron spectrum of argon recorded at the same conditions as the spectra discussed in this paper is presented in fig. 1. The ionisation limits of Ar (2P,,2 and 2P3,2) can be seen as intense peaks. The transmission function for the threshold electrons can be derived immediately from the profiie of the 2P,,2 peak. A background of 0.5Oh
I
I
Ar TPES I
I
I
15.9
15.95
h 2P3,215.7597
1’5.7
15.75
15.x
15.85 Encrgy(cV)
Fig. 1. TPE spectrum of Ar. Test of the resolution.
16
482
F. Merkt et al. /Chemical Physics 173 (I 993) 4 79-489
of the total ionisation can be seen at all frequencies. If it is subtracted from the TPE spectrum, one sees that electrons of 30 meV kinetic energies are still transmitted with 5Oh efficiency. This explains the presence of a weak line on the high energy side of the 2P,,2 limit that corresponds to electrons produced by spin-orbit autoionisation on the near-lying Rydberg state belonging to the series converging to the *P,,* limit. As a result, the lines whose intensity is smaller than 5% of the most intense peak in the TPE spectrum cannot be unambiguously assigned to ionic states, but might originate from autoionisation of a Rydberg state lying up to 30 meV above an ionic state. The dynamical range of intensities that can be probed by TPES therefore amounts to approximately 20. Whether weak lines can effectively be attributed to an ionic state must be checked by time-of-flight photoelectron spectroscopy as is illustrated in this work. The energy calibration of the TOF-PES was performed by measuring the TOF of electrons released from Ar as a function of the exciting wavelength. The TOF spectra of the first vibrational members of the 0: b state were also used in the calibration. The accuracy of the calibration is expected to be of the order of 10 meV for the lines that correspond to electrons emitted with less than 1 eV kinetic energy. The intensity of the peaks measured in the TOF spectra was determined by weighting the experimental data with the transmission function of the TOF spectrometer, which was determined from TOF photoelectron spectra of Ar. The intensity of the peaks is expected 0,’
to become more accurate for signals corresponding electrons emitted with increasing kinetic energy.
3. Results and discussion For the analysis, particular attention is given to four regions of the spectrum for which the results can be presented as follows. 3.1. The X ‘II, state of 0: in the region of the a “II,, state(15.4-17eV) A particularly striking feature of the TPE spectrum of 02 near the onset of the a 411Ustate is the long progression due to high vibrational members of the X 2Hs state. As can be seen from fig. 2, the two spin-orbit components of the X state are well resolved and the progression can be followed up to v+ = 26. Moreover, the intensity of the high vibrational components is strong enough to obscure the onset of the a state in the TPE spectrum. This is in sharp contrast to the He (I) [ 1 ] and He (II) [ 5 ] photoelectron spectra in which the vibrational progression in the X state stops approximately at U+ = 5, a consequence of vanishing Franck-Condon factors. The Ne( I) PE spectrum however is rather similar to the TPE spectrum, in that the X state is readily observed up to v+ > 20 [ 41. The non-Franck-Condon behaviour observed in the TPE spectrum can be attributed to the high density, in this spectral region, of autoionising Rydberg states that belong to series converging to the a and A states. Au-
TPES
X
Energy Fig. 2. TPES of Q between 15.4 and 17 eV. The spectrum overlaps the first vibrational members of the a state.
to
and
a states
(eV)
shows a very long vibrational
progression
in the X state, which partially
I? Merkt et al. /Chemrcal Physics 173 (1993) 475489
toionisation of these states to the high vibrational levels of the X state produces the electrons of low kinetic energy that constitute the TPE signal. The overlap between the high V+ members of the X state and low U+ members of the a state could, in principle, have some bearing on the interpretation of reaction dynamical experiments that rely on the TPEPICO technique to prepare 0: ions in the low vibrational members of the a state [ 26,271. For this reason, we have attempted to determine the percentA
JOO _,
300
_
200
_
loo_
O_
$
16.05
16.10
16.15
z
W
16.20
16.30
16.25
16.35
16.30
16140
483
age of the TPES signal that stems from the a and X states in the regions surrounding the v+ =O to v+ = 3 components of the a state. Details of the TPE spectrum in these regions can be seen in figs. 3a-3c. The percentages of X and a state obtained by fitting each profile with three Gaussians are summarised in table 1. The width assumed for the members of the X and a state was taken to be that measured from non-overlapping X and a lines in the TPE spectrum (40 and 80 cm-’ respectively) and that for the a state is the same width as measured for the v+ = 3,4 and 5 peaks which are separated from the X state transitions but represent an unresolved contribution of four spinorbit sublevels. The “deconvolution” procedure does not take into account the weak but wide wings that are observed on both sides of the bands and which are due to either a room-temperature contribution to the spectrum or to rotational branches associated with large core angular momentum changes. This is however not significant for the present analysis as these wings amount to less than 10% of the overall peak intensities. The X and a contribution presented in table 1 are supported by the observations that the intensities of those low members of the a state that are not overlapped by the X state correspond well to the intensities in the He(I) PE spectrum [ 5 ] and to calculated Franck-Condon factors and that, after deconvolution, the intensity of the u+ =O-3 components of the a state also corresponds to FranckCondon factors. The TPES intensities of the first vibrational members of the a state are shown in table 2. The values indicated in table 2 are normalised to the value of the v + = 4 state that is arbitrarily given a value of 100. The contrast between the regions above and below the a state is striking since Rydberg levels converging to the a and A state are seen to autoionise to the X Table 1 Percentage of the TPES stgnal which stems from the X and a states of 0: in the vicmity of the lowest four vibrational members of the a state
ENERGY/eV
Fig. 3. Details of the regions of the TPE spectrum of O2 in which high v+ members of the X state of 0: overlap the first member of the a state. (a) TPE spectrum in the region of the a (u+ ~0) and X (v+ = 2 1) states. (b ) TPE spectrum in the region of the a (v+ = 1) and X (v+ = 22) states. (c) TPE spectrum in the region ofthea (v+=2) andX (v+=23) states.
X(v+=2l),a(v+=O) X(v+=22),a(v+=l) X(v+=23),a(v+=2) X(v+=24),a(v+=3)
X state
a state
70% 55% 35% 15%
30% 45% 65% 85%
484
F. Merkt et al. /Chemical
Physics I:73 (I 993) 4 79-489
Table 2 Line intensities derived from the TPE spectrum of the a state of 0:. Aurelio Ferreira V+
0
1 2 3 4
Comparison with Franck-Condon factors from the thesis of L.F.
hw (eV)
hv (eV)
this work
calculated
Intensity (counts/s)
Intensity normalised
FCF normalised
16.102 16.227 16.350 16.471 16.590
16.101 16.221 16.350 16.470 16.589
115 430 850 I200 1430
8.0 36.1 59.4 83.9 100
7.9 28.9 51.9 84.2 100
state of 0: whereas Rydberg levels converging to the b state and situated above the a (v+ = 0) limit do not seem to do so. Inspection of the high-resolution photoionisation yield spectrum of Dehmer and Chupka in this region, i.e. 740-770 A, shows that the Rydberg levels are weak and diffuse [ 29 1. They are likely to be predissociated since Carlson observed a strong emission from excited neutral oxygen atoms in this region of the spectrum [ 28 1. 3.2. The b *t;; state of 0:
between 18 and 20 eV
An interesting question in this region is whether autoionisation leads to the observation of high vibrational levels of the b state as is true for the X state. The TPE spectrum of the b state is presented in fig. 4. The intensities of the first vibrational members up to U+ = 7 fall off quicker than predicted from FranckCondon factors as can be seen from table 3 (columns 4 and 5). The photoionisation spectrum in this re-
r
gion is dominated by several series of autoionising Rydberg states that converge to the first three vibrational members of the B state [ 291. These states autoionise predominantly to the b state [ 301, and the comparable geometry of the b and B states imply predominantly autoionisation with A (v+ =O). It is therefore not surprising that the low vibrational levels of the b state appear too intense compared to that of higher vibrational levels. The vibrational progression in the b state can be followed up to v+ = 18 even though the intensities of the members with v+ > 8 are very weak and appear rather irregular. The line positions measured in the TPE spectrum are compared in table 3 (column 2 and 3) with the values predicted using a Morse potential with w, = 1196.77 cm-’ and o&x, = 17.09 cm-’ from ref. [ 3 1 ] and an adiabatic ionisation potential of 18.17 1 eV [ 5 1. A second-degree polynomial fit of the data yields o,= 119 1.3 cm-’ and oar= 17.14 cm-’ which is in very close agreement. We calculate a dissociation energy of 2.576
02+ b 4zg---Y--------1-
-------
-~- - _I_r_---
1
I
“18
18.5
19.5 19 Energy (eV)
20.5
Fig. 4. TPE spectrum of the 0: b *Z; +-Or X ‘Z; transition. The vibrational progression in the b state can be followed up to v+ = 18.
F. Merkt et al. / Chemical Physics 173 (I 993) 4 79-489
485
Table 3 Line positions of the transitions to the different vibrational members of the b state. The values are compared with values calculated using a Morse potential II+
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
hv (eV) exp. 18.171 18.314 18.452 18.586 18.716 18.847 18.970 19.085 19.200 19.308 19.416 ( 19.626) 19.793 19.878 19.955 20.036 20.098
hv (eV) talc.
I (counts/s)
I normalised
FCF normalised
18.717 18.315 18.455 18.591 18.722 18.849 18.972 19.091 19.205 19.316 19.422 19.524 19.621 19.714 19.803 19.886 19.968 20.039 20.117
3700 2400 980 200 80 93 55 35 15 43 25
100 64.9 26.5 5.4 2.1 2.5 1.5
100 81.9 39.6 15.1 5.0 1.6 0.5
eV and a dissociation limit of 20.747 eV very close to the thermodynamic value of 20.7 eV. The observation in the TPE spectrum of transitions to these high vibrational members of the b state in the FranckCondon gap between the b and B states is somewhat surprising given the comment made above that autoionisation from the Rydberg states converging to the B state should be governed by a Au+ = 0 propensity rule [ 321, and should therefore not lend intensity to TPES lines corresponding to v+ > 8. A possible explanation for the presence of these lines could be that their intensity is gained from autoionisation from an as yet unidentified Rydberg series converging to the ionic state observed recently by Baltzer et al. [ 51 around 20.35 eV (see also sections 3.3 and 3.4 below). Alternatively, these electrons could be produced in a process similar to that of resonant autoionisation. In fact, the dissociation limit of 0; lies at 18.733 eV. Above this limit, the b and B states are known to be predissociated [ 331 so that the autoionisation of the Rydberg electron could occur while the molecular ion core dissociates, leading to a large change of geometry. Alternatively one may consider the autoionisation of repulsive Rydberg states with
low principal quantum number converging to the 3 *II” state whose vertical IP is observed at 23 eV [ 5,9 1. Evidence for such processes involving dissociative neutral states is given by the observation of 02+ B’C
19.6
20
P
20.4 20.8 Energy(eV)
21.2
21.6
Fig. 5. TPE spectrum of the 0: B *Z; to2 X ‘2; transition. The lower frequency part of the spectrum corresponds to the high frequency part of fig. 4. Note the presence of a weak additional progression in this region.
486
F Merkt et al. / Chemrcal Phystcs 173 (I 993) 4 79-489
0* autoionising lines in the PES of Cafolla et al. [ 341. Such atomic transitions result from autoionisation of excited oxygen atoms produced by the dissociation of the excited molecule. They are also seen in the present study, see section 3.4. Two other weaker progressions are also seen above 19.6 eV, they are identified with the help of the TOF-PE spectra and discussed in section 3.4. 3.3. The TPES in the region of the B ‘Z; 0: (20 and 21.5 ev)
0
1
2
3
I
5
/
b - STATE
6
state of
This region of the spectrum deserves special attention since Baltzer et al. [ 5 ] have recently and for the first time identified a new progression in the photoelectron spectrum of molecular oxygen. However, some doubt persists concerning the assignment of this progression. Two candidates have been proposed by Baltzer et al. [ 51: the 2Ag, which was first observed by Jonathan et al. [ 231 in the He(I) PES spectrum of 02, excited the ‘Ag state and the 2 *II” state which still remains to be identified. In ref. [ 5 1, the experiment was compared with an ab initio calculation of the potential energy surface of the 2 211u which predicted a vibrational spacing rather similar to that measured experimentally. However, the experimental binding energy was found to be some 0.66 eV higher than the theoretical prediction. A possible explanation for this discrepancy could be that the first vibrational members of the progression cannot be seen in the PE spectrum. If this is the case, they might be observable in the TPE spectrum, if they gain intensity by autoionisation. The threshold photoelectron spectrum of 0, in the region of the B state is displayed in fig. 6. Except for the v+ = 4 level which appears unexpectedly weak, the vibrational progression of the B state does not show significant intensity perturbations compared to that observed in the high-resolution He (I ) PES. In addition, all the lines observed by Baltzer et al. [ 5 1, and attributed to the new progression, can be seen in the TPES, with comparable intensities. The effect of autoionisation on the TPE spectrum of the X state between 15 and 17 eV differs considerably from that observed in the region of the B state between 20 and 2 1.5 eV. In the former case, the autoionisation lines are sharp and lead to non-Franck-Condon intensity distributions (see section 3.1). In the latter case, the
Fig. 6. Time-of-flight photoelectron spectra of 02, measured at different excitation frequencies between 18 and 20 eV: (a) 18.850 eV; (b) 19.090 eV; (c) 19.805 eV; (d) 19.945 eV; (e) 19.965 eV.
autoionisation resonances are particularly broad [ 29 ] and, while undoubtedly contributing to the threshold ionisation, do not seem to perturb the TPES intensities as drastically. Table 4 summarises the observed line positions and compares them with the values reported by Baltzer et al. [ 51. A second-degree polynomial fit of the data yields c&=805 cm-‘, 0~~ = 12.7 cm- ’ from which a dissociation energy of 1.58 eV is calculated with a Morse potential function. This yields a dissociation limit for this state of 2 1.93 eV close to the third thermodynamic limit 22.06 eV. One of the reasons to investigate the TPE spectrum in that region was to try and see whether the origin of the progression was effective at 20.35 eV and not at a lower frequency as suggested by the theoretical pre-
F. Merkt et al. /Chemical Physics 173 (1993) 4 79-489
Table 4 Line positions of the transitions to the different vibrational members of the weak progression observed in the vicinity of the B state. The values are compared with those obtained in the He(I) PE spectrum of Baltzer et al. [ 51. The assignment corresponds to that proposed by Baltzer et al. V+
0 1 2 3 4 5 6 7
hv (eV) this work
hv (eV)
20.350 20.448 20.545 20.636 20.724
20.35 1 20.450 20.544 20.637 20.726 20.810 20.890 20.968
20.890 20.973
Baltzer et al. [ 51
Indeed, it is well known that TPES signals can be detected in situations where no intensity is measurable by conventional PES. Unfortunately, the density of lines in the TPE spectrum between 19.6 and 20.3 is so high that it is not difficult to find a series of lines which would (accidentally or not ) fit with the rest of the progression. It is therefore justitied to ask whether some of them are due to autoionisation processes which release electrons of low kinetic energy. To clarify the situation, several TOFPES have been measured for a series of excitation frequencies between 19.8 and 20 eV. diction.
3.4. TOFphotoelectron spectra in the region just below 20 eV(19.6-20 ev) The high density of lines in the TPES just below 20 eV was noted at the end of the previous subsection. Some of these lines correspond to the high vibrational members of the b state and some of them might be assigned to the first members of the weak vibrational progression which partially overlaps the B state (see section 3.3). A series of TOF photoelectron spectra covering that region has been measured at several different excitation frequencies between 18 and 20 eV. The results are displayed in fig. 6. The TOF spectra a and b, which show the first vibrational levels of the b state, were used to calibrate the TOF spectrometer in energy. The spectra c-e were obtained by ionising the sample with photons of 19.805, 19.945 and 19.965 eV energy, respectively. In all these
487
TOF spectra, the progression in the b state can be unambiguously followed up to u+ = 7. Several lines which stem from autoionisation of excited 0 atoms produced by dissociation are indicated with dotted lines in fig. 6. These lines can be identified relatively easily in the TOF spectra as the kinetic energy of the photoelectrons does not depend on the excitation wavelength but solely on the energy difference between the metastable neutral atomic state and O+ ionic levels. The electrons emitted by atomic autoionisation have therefore the same time of flight in each spectrum and, as a result, appear at different energies after the TOF scale has been transformed to an energy scale. The occurrence of such atomic lines in the PES spectrum of O2 have been recognised previously by Cafolla et al. [ 341. A total of six lines are resolved in the TOF spectra between 19.6 and 20 eV and can be classified in two groups. Four intense peaks are almost equidistant and seem to belong to the same progression, their positions are reported in table 5. The spacing between these successive peaks (approximately 100 meV) does not correspond to the spacing of 114 meV, which would be expected if these lines were members of the progression discussed in the previous subsection. An assignment of these peaks to the first levels of the corresponding ionic state can therefore be ruled out. This spacing of 100 meV corresponds well to the spacing one would expect between two successive members of the b state in that region. However, the energies of the peaks appear to be displaced by approximately 20 meV from the position of the levels of the b state. In addition, if these peaks were to be assigned to vibrational members of the b state with u+ = 12-15, one
Table 5 Line positions of the transitions to the different vibrational members of the new weak progression observed below the B state lJ+
hv (eV) this work
0 1 2 3
19.643 19.743 19.836 19.924
0 1
19.78 19.89
hv (eV) Jonathan [ 231
19.79 19.90
488
F. Merkt et al. /Chemical Physm 173 (I 993) 479-489
ought to see z,+ = 11 as well in the TOF-PE spectra, which is not the case as can be seen from figs. 6c and 6e. These states seem therefore to form an additional progression starting at 19.64 eV with a vibrational spacing of approximately 100 meV. This is the first time this progression has been observed in a photoelectron spectrum. The reason for its occurrence in the TOF spectra presented here might be that it gains intensity from autoionisation. Two additional lines can be seen in the TOF spectra. Their intensity seems to depend strongly on the excitation frequency: the peak at 19.78 eV is barely noticeable in the TOF spectrum (e) but is the most intense in the TOF spectrum (c). Their frequencies ( 19.78 and 19.89 eV) correspond well to the frequencies of the first two members of the ‘4 state ( 19.79, 19.90) seen by Jonathan et al. [ 231. It cannot be ruled out at this stage that these two lines belong to the same progression as the lines observed by Baltzer [ 51 which overlap the B state (see section 3.3). We may now propose an assignment of the three weak transitions, reported in tables 4 and 5, based on the work of Beebe et al. [ 35 1. These authors carried out an ab initio calculation of all the bound states of 0:. The calculated ionisation potentials are within 0.2 eV of the experimental values for the known states. The two lines observed at 19.78 and 19.89 eV belong to the ‘A, state as discussed above. For the remaining four lines seen below 20 eV in the TOF-PES and which form a progression starting at 19.643 eV, we estimate a dissociation limit of 20.9 eV, close to the second dissociation limit of 0: at 20.7 eV. The “4 state with a calculated IP of 20.07 eV and o,= 82 meV seems an attractive candidate. Finally we find two candidates for Baltzer’s progression starting at 20.35 eV and having a vibrational constant of 100 meV: the 2’l-I” with an IP of 19.55 eV and w,= 120 meV as already proposed by Baltzer et al. [ 5 ] or the 2 ‘IIs state with an IP of 20.45 eV and w,= 120 meV. Since both states dissociate to the same limit at 22.06 eV the latter assignment may be considered as preferable but a definite conclusion will await further investigations.
4. Conclusions The high-resolution TPE-spectrum of partially cooled O2 has been measured between 12 and 22 eV and has revealed many new features of the 0: spectrum, some of them are very weak but nevertheless definitely identified as ionic levels. The two spin-orbit components of the X state are fully resolved and the vibrational progression in the X state can be followed up to v+ ~27. The intensity of the high v+ members of the X state is strong enough to obscure the onset of the a state. A deconvolution of the TPES spectrum in a and X contributions is performed in order to assist in the interpretation of reaction dynamical experiments which rely on the TPEPICO method to prepare 0: ions in the first levels of the a state. The vibrational progression in the b state of 0: is visible up to v+ = 18. The source of intensity of the vibrational components with v+ > 8 is not clear as the photoionisation spectrum in that region is dominated by autoionisation of Rydberg states belonging to series converging on the first vibrational levels of the B state [ 29,301 and that, due to the similar geometries of the B and b states, autoionisation from the B state to the b state should be governed by Au+ = 0 processes and should therefore not favour the production of high vibrational members of the b state. The intensity patterns observed for the B state are similar to those obtained by He(I) PES, indicating that autoionisation is not a dominant source of intensity in that region. An additional weak and yet unassigned vibrational progression which was first observed by Baltzer et al. [ 5 ] is clearly seen in the TPE spectrum. The present analysis confirms the results obtained in ref. [ 5 ] but we propose a different assignment. It is possible that this progression starts at a lower frequency than found by Baltzer et al. but the high density of the lines in the threshold photoelectron spectrum and the weakness of the lines which could be identified as the first members of this new progression prevent a definitive assignment. Further work on the isotopic ‘*O2 molecule is in progress to solve the problem. TOF photoelectron spectra have been measured in the energy region below 20 eV in order to confirm the assignment of some of the weak TPES lines. These spectra reveal a series of six lines between 19.6 and 20 eV. Four of these, with energies 19.643, 19.743,
F. Merkt et al. /Chemcal
19.836 and 19.924 seem to form an as yet unobserved progression. The intensities of the remaining two lines are very sensitive to the excitation wavelength and their position corresponds well to the energy of the first two vibrational levels of the *Agstate of 0: seen by Jonathan et al. [ 23 ] which is observed for the first time in the PES of normal Oz. Additional work on isotopes of O2 is currently in progress to provide definitive answers to the remaining questions. The use of high-resolution time-of-flight photoelectron spectroscopy (TOF-PES) in combination with threshold photoelectron spectroscopy (TPES) has been shown to be efficient in extending the dynamic range of the TPES method to low intensity lines, i.e. less than 2.5% of the main lines, which could not otherwise be unambiguously identified with ionic levels because of autoionization.
Physics 173 (1993) 479-489
489
[7] P.M. Guyon, J. Hepburn, T. Weng, F. Heiser Reynolds, Phys. Rev. Letters 67 (1991) 675. 181 R. Stockbauer,
Advan. Mass. Spectrom.
[9] P.M. Guyon and I. Nenner,
8 (1980)
Appl. Opt. 19
[ 101 J.M. Ajello and A. Chutjian,
and 79.
( 1980) 4068.
J. Chem. Phys. 71 (1979)
] 111 P.M. Guyon, R. Spohr, W.A. Chupka Chem. Phys. 65 ( 1976) 1650.
D.
1079.
and J. Berkowitz,
J.
[ 121 W.B. Peatman, J. Chem. Phys. 64 (1976) 4368. [ 131 T. Baer and P.M. Guyon, J. Chem. Phys. 85 (1986)
4765.
[ 141 T. Baer, P.M. Guyon, I. Nenner, A. Tabche-Fouhailt, Botter, L.F.A. Ferreira (1979) 1585. ]15
P.M. Guyon, (1983)
R.
and T.R. Govers, J. Chem. Phys. 70
T. Baer and I. Nenner,
J. Chem.
Phys.
78
3665.
116 W.A. Chupka and E. Eyler, J. Chem. Phys. 88 ( 1988) 3032. M. Sander and E.W. Schlag, Chem. ]17 K. Miiller-Dethlefs, Phys. Letters
112 (1984)
291.
[ 181 F. Merkt and T.P. Softley, J. Chem. Phys. 96 (1992)
4149.
[ 19 ] F. Merkt and T.P. Softley, Phys. Rev. A 46 ( 1992) 302. [20] J. Hepburn
et al., to be published.
I 1F. Merkt and P.M. Guyon, to be published. 122] R.N. Dixon and S.E. Hull, Chem. Phys. Letters 3 ( 1969) [2
Acknowledgement We thank the staff of LURE for the financial and technical support of this experiment and for operating the super-AC0 storage ring. We thank F. Heiser and 9. Alonsi for helping us with some of the data handling. We are especially grateful to 9. Lagarde and P. Martin who were in charge of the 3 m monochromator installation on the SA 63 beam line and we thank the Ministhe des Affaires Etrangeres, France, for financial support to one of us (FM).
367. A. Morris, M. Okuda, K.J. Ross and D.J. ]23 ] N. Jonathan, Smtth, J. Chem. Sot. Faraday Trans. II 70 (1974) 1810. ]24 ] M.R. Viard, 0. Dutuit, 0. Atabek and P.M. Guyon, J. Chem. Phys. 93 (1990) 8881; I. Powis, 0. Dutmt, M. Richard-Viard and P.M. Guyon, J. Chem. Phys. 92 ( 1990) 1643. [25] R. Tonkm, Letters
J.W. Winmczek
164 (1989)
and M.G. White, Chem. Phys.
137.
[26] K. Tanaka, T. Kato, P.M. Guyon Phys. 92 (1982) 1643.
J. Chem. Phys. 60 (1974)
[29] P.M. Dehmer and W.A. Chupka,
[ 1] 0. Edqvist, E. Lindholm, L.E. Selin and L. Asbrink, Physica Scripta 1 (1970) 25. [2] J.L. Gardner and J.A.R. Samson, J. Chem. Phys. 62 (1975) 4460. [ 31 H. van Lonkhuyzen and C.A. de Lange, J. Electron Spectry. 27 (1982) 255. [4] J.A.R. Samson and J.L. Gardner, J. Chem. Phys. 67 (1977) 755. [ 51 P. Baltzer, B. Wannberg, L. Karlsson, M. Carlsson-Gothe and M. Larsson, Phys. Rev. A 45 (1992) 4374. [6] P. Morin, I. Nenner, P.M. Guyon, 0. Dututt and K. Ito, J. Chim. Phys. 77 ( 1980) 78; P. Morin, I. Nenner, P.M. Guyon, L.F.A. Ferretra and K. Ito, Chem. Phys. Letters 75 ( 198 1) 1129.
J. Chem.
[ 27 ] C. Alcaraz, 0. Dutuit and P.M. Guyon, to be published. [28] R.W. Carlson,
References
and I. Koyano,
2350.
J. Chem. Phys. 62 (1975)
4525. [30] A. Tabche-Fouhatlt, I. Nenner, P.M. Guyon Delwiche, J. Chem. Phys. 75 (1981) 1129.
[ 311 K.P. Huber molecular molecules
and
G. Herzberg,
spectra
structure, Vol. IV. Constants of (Van Nostrand, New York, 1979).
[32] P. Morin, I. Nenner, Delwiche,
Molecular
and
and
diatomtc
M.Y. Adam, M.J. Hubin-Franskin,
H. Levebvre-Brian
Phys. Letters 92 (1982)
and A. Giusti-Suzor,
J.
J.
Chem.
609.
[ 33 ] P.M. Guyon, T. Baer, L.F.A. Ferreira, I. Nenner, A. TabcheFouhaile,
R. Botter and T. Govers,
LI, and references
J. Phys. B 11 (1978)
therein.
[ 341 A.A. Cafolla, T. Reddish and J. Comer, J. Phys. B 22 ( 1989) L273.
[ 35 ] N.H.F. Beebe, E.W. Thulstrup Phys. 64 (1976)
2080.
and A. Andersen,
J. Chem.