Autoionisation in polyatomic molecules

Transcription

1 J. Phys. B: At. Mol. Phys. 14 (1981) L67-L71. Printed in Great Britain LE ITER TO THE EDITOR Autoionisation in polyatomic molecules J P Conneradett, M A BaigSII and S P McGlynnOI t Blackett Laboratory, Imperial College, London SW7 2BZ, England j: Physikalisches Institut, Universitat Bonn, Nussallee 12, 53 Bonn, West Germany 8 Choppin Chemical Laboratories, Louisiana State University, Baton Rouge, Louisiana 70803, USA Received 8 December 1980 Abstract. A brief discussion of the de-excitation probabilities associated with autoionisation, vibrational relaxation and predissociation channels suggests that the autoionisation route can dominate the others in high-n Rydberg molecular states. Experimental investigations at high resolution and energy verify this conclusion and demonstrate that the variety of profile shapes may be as rich as or even richer than that for atoms. Autoionisation events in a complex molecular system must compete with other de-excitation channels which may possess comparable or even larger cross sections. Of these, the most effective are, (i) vibrational deactivation processes and (ii) predissociative events. Since type (i) events can occur within one or a few vibrational periods, 14 the half lives for such decay processes may lie in the range k k 10- s. Type (ii) events are a subclass of type (i), the difference being that type (ii) events occur on a potential surface which is unbound with respect to at least one normal mode, whereas type (i) events are mediated by bound vibrational processes. As a result, type (ii) events are restricted to one vibrational period and they are expected to lie in the same (or 14 even shorter) time range as type (i) events: that is, ~~3 10- s. Since autoionisation events in atoms (Fano 1961) are typically of half life T~ about s and since there is every reason to suppose that they are comparable in molecules, it follows that such events need neither dominate nor, indeed, even occur to any significant extent in the de-excitative channelling of the excited electronic states of polyatomic molecules. A further factor in favour of type (i) and (ii) events in polyatomics is caused by the linear increase in the number X of these channels: K = 3N - 6, where N is the number of atoms in the polyatomic molecule. Thus, K = 9 for methyl iodide and X = 30 for benzene. On the other hand, the number of autoionisation channels in a given energy range is not an obvious function of molecular complexity and it may increase slightly, remain constant, or even decrease with increasing N. The above arguments suggest that autoionisation is not a probable molecular relaxation process. As a result, the observed autoionisation structures in molecular hydrogen and other diatomics are frequently considered to be exceptional. It is reasoned that H2, together with all diatomics, is unique in that it possesses only one totally-symmetric normal mode and no non-totally-symmetric normal modes, and that 11 Alexander von Humboldt Fellow. 1 Alexander von Humboldt Senior US Scientist supported by the US Department of Energy /81/ The Institute of Physics L67

2 Letter to the Editor the observation of autoionisation in a vibration poor diatomic is no guarantor of its occurrence in more complex (i.e. vibration rich ) molecules. In the present letter, we advance an alternative point of view which suggests that autoionisation can be important in the de-excitation channelling of certain excited states in polyatomic molecules. The similarity between autoionisation and predissociation in that both involve discrete/continuum state interactions, the former electronic and the latter vibrational, has been discussed by Herzberg (1971) and, from a theoretical standpoint, by Fano (1977). In this view, the competitive stance of predissociation vis-a-vis autoionisation must rely on higher vibrational state densities and severe Born-Oppenheimer breakdown. Now, it is known that the density of vibrational structure associated with molecular Rydberg states tends to decrease with increasing n, and it is not uncommon to refer to such states as quasi-atomic. If, as seems reasonable, such attribution suggests an uncoupling of high-n Rydbergs from the atomic framework of the molecule, then it also implies a relative decrease in Born-Oppenheimer breakdown and in predissociative cross sections. Thus, one may not discount high-n autoionisation events even in polyatomic molecules. In the light diatomics the situation is different, and broadening by autoionisation (or preionisation as it is often called in this context) has been known for a long time in H2 (Beutler et a1 1936, Beutler and Junger 1936a, b) and in N2 (Hopfield 1930a, b, c). The case of HZ has, of course, been the most extensively studied and the effects of autoionisation have been very clearly established (Jeppensen 1954, Diebeler et a1 1965, Berry 1966, Chupka and Berkowitz 1969, Sroka 1969, Herzberg and Jungen 1972, Breton et a1 1980, etc). There has also been recent work on autoionisation in 0 2 (Codling et a1 1980) and other diatomics (Nakamura et a1 1980, etc). We have verified experimentally that autoionisation events for high-n states in polyatomic molecules above the first ionisation potential are more common than not. In our investigations of photoabsorption spectra at the Synchrotron Radiation Laboratory in Bonn, we have had the dual benefits of high resolution (zto.003 A) and high energy (A < 1200 A). Very long path lengths (10 m) and low pressures were also used, which favour the observation of sharp lines (for a description of our system cf Connerade er a1 1980). We have been able to measure band profiles of autoionising molecular transitions undistorted by apparatus artifacts or limitations. Thus, the content of this work consists of the presentation of a few exemplary spectra. The molecules chosen have well developed Rydberg series-in order to satisfy the high-n criterion-and, in addition, each is representative of a broad class of polyatomics-in order to satisfy our claim for generality. The representative of the large class of aromatics is benzene. The long Rydberg series shown in figure 1 was previously thought to terminate abruptly in a continuum onset around 11.4 ev (cf Ockenga er a1 1980), on the basis of observations at lower resolution than reported here. However, as is clear from figure 1, this is not the case: the series can be followed up to a broad feature with which it interacts and, beyond this broad feature, to high values of n. The resulting series perturbation closely parallels Rydberg series/broadband interactions observed e.g. in atomic thallium (Connerade 1978) and is a clear cut manifestation of autoionisation. The representatives of the class of saturated hydrocarbons are methane type molecules in which we have substituted I or Br atoms in order to develop long Rydberg series (Wang et a1 1977). Both spectra contain clear examples of autoionisation, the (2E1/2)nd series exhibiting asymmetries fully comparable with the (2P1/2)nd series of

3 letter to the Editor L69 n h (A) Figure 1. The absorption spectrum of C6H6 showing the Rydberg series converging on a 3e2, limit above the first ionisation potential and perturbation by a broad feature described in the text. Note the asymmetries of the overlapping profiles. atomic Xe (Huffman et al 1963). These asymmetries are caused by autoionisation into the (2E3/2)Ed continuum. From the standpoint of the present letter, however, the most conclusive example is provided by the ('E112)nd series in CH3Br (see figure 2). The upper members of this series impinge on and interact with a very broad interloping feature. Not only are asymmetric Beutler-Fano profiles observed, but they are seen to reverse (i.e. q changes sign) as the point of maximum interaction between the series and the broad features is traversed. This clear manifestation of autoionisation is again fully comparable with observations for atoms (Connerade 1978) iim 1179 h (A) Figure 2. The absorption spectrum of CH3Br showing the (2El/2)nd series perturbation and q reversal described in the text.

4 L70 Letter to the Editor Finally, in figure 3, we show a section of the spectrum of a typical small inorganic molecule, HzO, just above the first ionisation potential. It is significant that broad asymmetric profiles occur amongst the members of a rovibronic progression which lies embedded in the continuum close above the ionisation threshold. We have illustrated in the figure how the two members lowest in energy can be fitted to the Fano (1961) formula for an isolated resonance. While this procedure is clearly approximate in the presence of overlapping transitions, it does provide a qualitative explanation for the appearance of the spectrum. r A (a) Figure 3. This figure illustrates possible autoionisation in H20. The dotted curves are approximate fits calculated from the Fano (1961) formula for an isolated resonance (with an assumed background level represented by the full line). The interpretation does not purport to be quantitative but merely illustrates the point made in the text. The fact that autoionisation is conspicuous below the clear rovibronic progression and disappears towards higher energies is presumably attributable to some change in the Born-Oppenheimer conditions which determine the coupling between the nuclear and electronic motions. 6 I In sum, we conclude that: (a) Autoionisation is common in the Rydberg states of a polyatomic molecule. (b) Autoionisation seems improbable for highly excited intravalence states, because of the many alternative damping mechanisms. (c) Autoionising series in polyatomic molecules maintain integrity through interloping resonances and exhibit q reversals just as in the atoms. (d) Beutler-Fano profiles are not much distorted by rotational excitation in polyatomics. (e) The Born-Oppenheimer approximation is evidently valid for many long Rydberg series in polyatomic molecules. (f> The l/n*3 dependence of 'core induced' autoionisation (Fano and Cooper 1965) is the only aspect of autoionisation which is difficult to carry over to molecules since it manifests itself primarily among low-n atomic states.

5 Letter to the Editor L7 1 The authors are grateful for useful comments received on an earlier version of the text from Dr G W Findley of Louisiana State University. The present work received financial support from the SRC and the DFG. References Berry R S 1966 J. Chem. Phys Beutler H, Deubner A and Junger H D Phys Beutler H and Junger H D 1936a 2. Phys b Z. Phys Breton J, Guyon P M and Glass-Maujean M 1980 Phys. Rev. A Chupka W A and Berkowitz J 1969 J. Chem. Phys Codling K, Parr A C, Ederer D L, Cole B E, Stockbauer R, West J B and Dehmer J L 1981 J. Phys. B: At. Mol. Phys. in press Connerade J P 1978 Proc. R. Soc. A Connerade J P, Baig M A, McGlynn S P and Garton W R S 1980 J. Phys B: At. Mol. Phys. 13 L705 Diebeler V H, Reese R M and Krauss M 1965 J. Chem. Phys Fano U 1961 Phys. Rev Colloq. Int. on Atomic and Molecular States Coupled to a Continuum, Highly Excited Atoms and Molecules, Aussois (Paris: Editions du CNRS) Fano U and Cooper J W 1965 Phys. Rev. 137 A1364 Herzberg G 1971 Topics in Modern Physics (Colorado University Press) p 191 Herzberg G and Jungen Ch 1972 J. Mol. Spectrosc Hopfield J J 1930a Phys. Rev b Phys. Rev c Astrophys. J Huffman R E, Tanaka Y and Larabee J C 1963 J. Chem. Phys Jeppensen C R 1954 Phys. Rev Nakamura M, Morioka Y, Iida Y, Masuko H, Hayaishi T, Ishiguro E and Sasanuma M 1980 Proc. 6th Int. Conf. on VUVRadiation Physics vol2 (Charlottesville: University of Virginia) paper 23 Ockenga K E, Gurtler P, Hasnain S S, Saile V and Koch E E 1980 Proc. 6th Int. Conf. on VUVRadiation Physics vol 2 (Charlottesville: University of Virginia) paper 80 Sroka W 1969 Phys. Lett. 28A 784 Wang H T, Felps W S and McGlynn S P 1977 J. Chem. Phys