Photoabsorption, photoionization, and neutral-dissociation cross sections of simple hydrocarbons in the vacuum ultraviolet range

Journal of Electron Spectroscopy and Related Phenomena 123 (2002) 225–238 www.elsevier.com / locate / elspec

Photoabsorption, photoionization, and neutral-dissociation cross sections of simple hydrocarbons in the vacuum ultraviolet range a a b a, Kosei Kameta , Noriyuki Kouchi , Masatoshi Ukai , Yoshihiko Hatano * a

b

Department of Chemistry, Tokyo Institute of Technology, Meguro-ku, Tokyo 152 -8551, Japan Department of Applied Physics, Tokyo University of Agriculture and Technology, Koganei-shi, Tokyo 184 -8588, Japan Received 22 November 2001; accepted 29 January 2002

Abstract The absolute photoabsorption cross sections, st , of CH 4 and n-C 4 H 10 have been measured in the photon energy range of the valence electrons using a double ionization chamber equipped with a metallic thin film window and synchrotron radiation as a continuous-wavelength light source. The absolute photoionization quantum yields, h, of CH 4 have been also measured, from which the photoionization cross sections, si , and neutral-dissociation cross sections, sd , are obtained. The values of st and h of C 1 –C 4 normal alkanes including our previous results for C 2 H 6 and C 3 H 8 have been compared in detail with those by the dipole-simulation method using the virtual photons. The gross features of the st and h values of these alkanes are discussed in terms of those as functions of the photon energy and the number of C atoms in an alkane molecule.  2002 Elsevier Science B.V. All rights reserved. Keywords: Superexcited state; Photoabsorption cross section; Photoionization cross section; Neutral-dissociation cross section; Alkane; Synchrotron radiation; Vacuum ultraviolet range

1. Introduction

such an initial interaction of a photon with a molecule are schematically expressed as follows [1]

The investigation of the interaction of photons with molecules, in particular the cross section data on their interaction, i.e. those on photoabsorption, photoionization, and photodissociation of molecules, is of great importance in fundamental sciences [1–7] and is greatly needed in applied fields [8–10]. The absorption of a single photon by a molecule causes an electronic transition, and the major processes in

AB → AB 1 1 e 2

(1)

→ AB9

superexcitation

(2)

→ AB*

excitation

(3)

AB9 → AB 1 1 e 2 autoionization →A1B

*Corresponding author. Department of Molecular and Material Sciences, Kyushu University, Kasuga-shi, Fukuoka 816-8580, Japan. Tel.: 181-92-583-7552; fax: 181-92-583-7557. E-mail addresses: [email protected] (K. Kameta), [email protected] (Y. Hatano).

direct ionization

→ AB0

neutral dissociation processes other than (4) and (5).

(4) (5) (6)

When a molecule AB receives energy which is

0368-2048 / 02 / $ – see front matter  2002 Elsevier Science B.V. All rights reserved. PII: S0368-2048( 02 )00022-1

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larger than its first ionization potential, Ip, AB may be directly ionized and may be excited to form a superexcited molecule AB9, which can ionize or dissociate into neutral fragments. Since the photoionization quantum yield, h, is defined as the ratio of the photoionization cross section, si , to the photoabsorption cross section, st , i.e. h 5 si /st , the value of 1 2 h shows the relative importance of neutraldissociation processes in the total decay channels of AB9 because the process (6) seems not to be important in the decay of such extremely highly excited states. As one of the processes (6), ion-pair formation, AB9 → A1 1B 2 , has been extensively studied to clarify the dynamics of superexcited molecules, although its cross section is much smaller than those of the processes (4) and (5) [11–13]. The value of h may therefore be smaller than unity in the energy ranges above the first Ip, which means that the molecules excited into this energy range are not always ionized but use their energy for processes other than ionization. Once the values of st and h have been measured, the values of the si and neutraldissociation cross sections, sd , are obtained from the following equations, si 5 st 3 h and sd 5 st 3 (1 2 h ). The experimental methods to study the spectroscopy and dynamics of molecular superexcited states as well as the primary processes, i.e., (1)–(6), are classified depending on the method to produce superexcited states as follows [1–10],[24] 1. 2. 3. 4.

Electron impact spectroscopy, Discharge-lamp photon impact spectroscopy, Laser photon impact spectroscopy, Synchrotron radiation photon impact spectroscopy, and 5. Coincident electron-energy-loss spectroscopy. The method (1) is further classified as follows [1,14–19,29]: (i) Measurements of the absolute cross sections for the production of excited fragments by fast electron impact, (ii) Measurements of the absolute cross sections for molecular photoabsorption and photoionization processes by fast electron impact, and (iii) Translational spectroscopy of dissociation fragments by electron-impact excitation of molecules. Electron beams and discharge-lamp photons were

mainly used in the 1960s and 1970s, and are still used for this purpose, whereas since the 1980s laser multiphotons and synchrotron radiation (SR) photons have also been used. Details of the electronic states of superexcited molecules at least in the opticallyallowed states and the mechanism of their autoionization and dissociation have been quickly clarified mainly by means of SR as an excitation source. The coincident electron-energy-loss spectroscopy has been recently developed as a new experimental method for studying molecular superexcited states in optically-forbidden states [20–25]. The spectroscopy and dynamics of the formation and decay processes of molecular superexcited states, in particular the neutral-dissociation process, have been recently surveyed comprehensively by the present authors’ group [1–10,24,26,27]. It is concluded in these reviews and feature articles that superexcited states are molecular Rydberg and nonRydberg states built on each ion state, which are vibrationally / rotationally, doubly, or inner-core excited. The dissociation dynamics as well as the products of dissociation of these states are very different from those of the lower states excited below the ionization thresholds. Another important conclusion is that molecules are, in general, not always ionized at energies above the first Ip. It has been found that neutral dissociation is unexpectedly important (i.e. h , 1) when molecules absorb energy in certain limited energy regions above their ionization thresholds. In addition to the investigation of the spectroscopy and dynamics of molecular superexcited states, to obtain the absolute cross sections for photoabsorption, photoionization, and neutral-dissociation of molecules is of also great importance in both fundamental and applied sciences. From this viewpoint, the methods (1) (ii) and (4) are of particular importance in playing complementary roles with each other. In the method (4) SR is used as a real photon source, while in the method (1) (ii) a fast inelastically scattered electron with vanishingly small momentum transfer is used as a virtual-photon source. Brion and his co-workers have extensively applied the method (1) (ii) using dipole (e, e), (e, 2e), and (e, e1ion) spectroscopies to a variety of molecules [16–18,28,29]. On the other hand we have been using the method (4) to measure the above

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three cross sections as described later. There have been comprehensive surveys of the comparative studies between the real- and virtual-photon experiments [1–4,10,18,29]. They are briefly summarized as follows. In electron impact experiments of molecules, extrapolation of energy loss intensities for incident electron energies much higher than 10 2 eV to zero momentum transfer should give, according to the Born–Bethe theory, the corresponding optical oscillator strength [30]. An alternative approach is to use very high impact energies (several keV) and zero degree scattering angle, such that the momentum transfer is negligibly small. This circumvents the need for any extrapolation to zero momentum transfer. Brion and co-workers [16–18,28,29] have extensively used and applied such an experimental approach using fast electrons as the virtual-photon source, which van der Wiel had earlier named ‘the poor man’s synchrotron’. Brion et al. [18,29] have pointed out some expected characteristics of the use of SR in comparison with virtual photons in studies of the photoionization and photoexcitation of molecules and made clear some necessary assumptions to virtual photons instead of real photons. It should be noted here, as described in detail below, that these two methods, real- and virtual-photon experiments, have complementary roles with each other in the investigation of the dynamics of superexcited molecules as well as in the understanding of essential features of the interaction of photons with molecules [4]. The fast electron scattering (dipole) approach to real-photon experiments using the virtual-photon source is called the electron impact dipole-simulation method. The dipole (e, e) method simulates total photoabsorption while coincidence techniques have been developed for measurements of the electronic partial photoionization (dipole (e, 2e)) and ionic

227

photofragmentation (dipole (e, e1ion)) cross sections. These virtual-photon techniques are summarized in Table 1 [18], in comparison with those used in real-photon experiments. These simulation techniques have provided a large body of data in comparison with photon experiments of absolute cross sections [18,29]. The photon experiments using SR, in which a lot of progress in experimental techniques is being made, have been compared with the simulation measurements [1]. Two examples, H 2 S and SiH 4 , are discussed below. The absolute values of st for H 2 S were measured earlier [31] using low resolution dipole (e, e) spectroscopy, DE51 eV, in the equivalent photon energy range up to 90 eV and were compared with those obtained from SR experiments [32], providing good agreement in the values of st between the two different experiments. This confirmed that the necessary assumptions, such as the S(0) (i.e. Thomas– Kuhn–Reiche) sum-rule normalization used in the simulation experiments, were reasonable at least for this molecule. This comparison also clearly illustrates a large difference in the wavelength- or energy-resolution between the two experiments. It should be noted that comprehensive studies of the photoabsorption of H 2 S had since been carried out [72,73] with the high resolution dipole (e, e) method [28,29,33–37] which provides an energy resolution of 0.048 eV fwhm. In the virtual-photon experiment for H 2 S [31], the value of the photoionization quantum yield was obtained indirectly from the sum of the cross sections for the partial (molecular and dissociative) photoionization divided by the total photoabsorption cross section with the assumption that the photoionization quantum yield is unity in the energy range above | 20 eV. There have been observed frequently sizable discrepancies in the photoionization quantum yield curves for molecules as a function of the photon energy obtained between

Table 1 Photon and electron impact experiments [18] Photon experiment

Equivalent electron-impact experiment

Total photoabsorption Total photoionization Photoelectron spectroscopy

Electron-energy-loss spectroscopy, dipole (e, e) Dipole (e, 2e) or (e, e1ion) (from sums of partials) Electron energy loss–ejected electron coincidence, dipole (e, 2e) Electron-ion coincidence, dipole (e, e1ion)

Photoionization mass spectrometry

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in the virtual-photon experiments and in the realphoton experiments [4,5]. However, the virtualphoton experiment shows some advantages over the real-photon experiment in that the gross features of photo-absorption, -ionization, and -ionic fragmentation of molecules are much more easily obtained over a broad energy range [4,5,18,29]. Another important advantage of the virtual-photon experiments over real-photon experiments, particularly in st measurements based on the Beer–Lambert law, is that the former does not suffer from the problem of an underestimate of the photoabsorption cross section for sharp structures in the spectrum where the photon bandwidth is comparable to or wider than the natural line width [30,33]. In SR experiments in the vacuum ultraviolet (VUV) region, e.g., at 80.0 nm (515.5 eV), an energy resolution of 20 meV which corresponds to a wavelength resolution of 0.1 nm is easily achieved, while in the simulation experiments the energy resolution is 0.048 eV (high resolution dipole (e, e)) and 1 eV (low resolution dipole (e, e)). In the case of SiH 4 , a marked discrepancy existed in the absolute values of st between the earlier virtual- and real-photon experiments [4,5,38–41]. The reasons for this are now well understood [28,29] and in a recent high-resolution dipole (e, e) experiment of SiH 4 [28], good agreement is obtained with the real-photon SR experiment [41]. The former discrepancy was ascribed to the interaction of target gas with the oxide cathode of the electron gun which affected the results in the case of a few reactive gases in the simulation experiment [28]. This effect was eliminated by installing a further stage of differential pumping [29,74]. In a further development the S( 2 2) sum rule analysis has been proposed as an alternative normalization procedure for the scale determination of st [29]. Although this simulation experiment has still a lower energy resolution at lower photon energies ( # 20 eV) in the st curve, it has a distinctive advantage in that it shows the gross features of photo-absorption, -ionization, and -ionic fragmentation of a molecule over a very wide energy range. In real-photon experiments we made significant improvement on our apparatus as follows. In the incident photon energy range higher than the LiF cutoff at 11.8 eV (105 nm), only a limited number of the measurements of the absolute st and h had been

carried out with insufficient accuracy because of the lack of appropriate window materials and a continuous-wavelength light source. Such an earlier situation of real-photon experiments has been critically surveyed in some review articles [1–5]. We have overcome these difficulties by using metallic thin films as window materials and SR as a light source, and we have directly measured with sufficient accuracy the absolute st and h values of some polyatomic molecules such as SiH 4 [41], Si 2 H 6 [42], SiX 4 (X5F, Cl, CH 3 ) [43], CH 3 OCH 3 and CH 3 CH 2 OCH 3 [44], C 2 H 2 [45], C 2 H 6 and C 3 H 8 [46], and cyclo-C 3 H 6 [47] in the vacuum ultraviolet range. In the present investigation, we have measured the absolute values of st , si , sd , and h of CH 4 (methane) and n-C 4 H 10 (normal butane) in the incident photon energy range of 10–24 eV (52–124 nm). These values of st and h together with those for C 2 H 6 (ethane) and C 3 H 8 (propane) reported by our group [46] are compared with corresponding quantities obtained by the dipole-simulation method [54,66] from the viewpoints mentioned above. We have also aimed at obtaining an overview of the interaction between the VUV photons and valence electrons in normal alkanes, Cn H 2n12 (n 5 1–4). In Fig. 1 the ionization potentials of the valence electrons in each of these alkane molecules are indicated to show how the outer and inner valence orbitals associated with C 2p and 2s orbitals, respectively, locate in energy [50–53].

2. Experimental The experimental setup was described in detail in our previous paper [41]. Briefly, continuum synchrotron radiation from a 2.5 GeV positron storage ring at the Photon Factory, the dedicated facility at the Institute of Materials Structure Science, as a continuous-wavelength light source was dispersed by a 3 m normal incidence monochromator equipped with a 2400 lines / mm grating (for CH 4 ) or a 1 m Seya monochromator with a 1200 lines / mm grating (for n-C 4 H 10 ). The photoabsorption cross sections, st , and photoionization quantum yields, h, were measured using a cylindrical type double ionization chamber equipped

K. Kameta et al. / Journal of Electron Spectroscopy and Related Phenomena 123 (2002) 225 – 238

Fig. 1. Ionization potentials of the valence electrons of CH 4 , C 2 H 6 , C 3 H 8 , and n-C 4 H 10 [50–53], which are the vertical ionization potentials of the ionic states produced with the removal of an electron from each valence orbital. The point groups for the molecules are also shown as well as the notation of the orbitals based on them.

with a metallic thin film or an LiF filter as window materials [41,46]. The photoionization cross sections, si , and neutral-dissociation cross sections, sd , were obtained from the st and h values, following the equations, si 5 st 3h and sd 5 st 3(1 2 h ), as mentioned in Section 1. Monochromatized light entered into the ionization chamber through an Sn thin film for the incident photon energy range of 16.8–24 eV (74–52 nm), and through an In thin film for the range of 11.3–17.2 eV (110–72 nm). The thickness of these films is approximately 100 nm (ULVAC Co.). Alternatively a 2 mm LiF window was used for the photon energy range lower than 11.8 eV ( $ 105 nm). These windows enabled exact measurements of the absolute values of st and h by eliminating the higher order components in the dispersed light and also by preventing the sample gas effusion back into the optical path. The values of st in the energy range below the first ionization potential were measured by the photoattenuation method using the ionization chamber as a conventional gas cell. Incident photon intensities were monitored with a gold mesh VUV detector, of

229

which absolute efficiencies were calibrated using the values of h for rare gases (Ar or Xe), i.e., h 51. Research grade CH 4 with a purity of 99.999% and n-C 4 H 10 with a purity higher than 99.8% were used without further purification. The gas pressures of 15–150 mTorr in the ionization chamber were measured by a capacitance manometer (MKS Baratron, model 310CHS-1 or 127A) and the effect of thermal transpiration was corrected using the reported equation [48,49]. The wavelength scale of the incident photon was calibrated using the window resonances by excitation of the 5s electron in Xe, resonances between the 2 P3 / 2 and 2 P1 / 2 thresholds of Xe, and window resonances by excitation of the 3s electron in Ar [41]. A wavelength scan-step of 0.02 nm (approximately 6 meV/ step at 20 eV incident photon energy) was used with a bandpass of 0.1 nm (32 meV width at 20 eV incident photon energy) for CH 4 , and the wavelength scan-step was 0.1 nm with a bandpass of 0.36 nm (116 meV at 20 eV) for n-C 4 H 10 . The systematic errors in the photoabsorption cross sections obtained in the present experiment were attributed mainly to those of current meters and a capacitance manometer. The total experimental errors in the photoabsorption cross sections are, thus, estimated to be within approximately 63%.

3. Results and discussion

3.1. CH4 (methane) 3.1.1. Photoabsorption, photoionization, and neutral dissociation cross sections, and photoionization quantum yields The electron configuration of the ground state CH 4 in T d symmetry is [50–53]: (1a 1 )2

(2a 1 )2 #%"!%$ inner valence

(1t 2 )6 . #%"!%$ outer valence

All the ionization potentials of the valence electrons are within the present incident photon energy range of 10–24 eV (see Fig. 1). This means that most of the ionization and superexcitation processes of the valence electrons are expected to contribute to the photoabsorption, photoionization, and neutral-dis-

230

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Fig. 2. The photoabsorption (st ), photoionization (si ), and neutral-dissociation (sd ) cross sections of CH 4 as a function of the incident photon energy measured in this experiment with a bandpass of 0.1 nm, which corresponds to the energy width of 32 meV at the incident photon energy of 20 eV. The vertical ionization potentials of the ionic states involved are also indicated by the vertical bars [53].

sociation cross sections in this energy range. Fig. 2 shows the absolute values of st , si , and sd of CH 4 as a function of the incident photon energy obtained in the present experiment. Those of st are again plotted in Fig. 3 to compare them with the results obtained

by other methods: the high resolution dipole (e, e) method [54], the low resolution dipole (e, e) method [54], the method with a double ionization chamber without windows and an Ar discharge lamp [55], the dipole-simulation method [56], the method with a double ionization chamber with windows / SR [57], and the photo-attenuation method without windows using discharge lamps [58]. All the st curves in Fig. 3 show a broad peak around 13.5 eV and they are in quantitatively good agreement with each other considering uncertainties in their absolute magnitudes, except for a much earlier result by Metzger and Cook [58]. Let us examine the present photoabsorption cross sections in terms of the Thomas–Kuhn–Reiche sum rule for the oscillator strength distribution which is proportional to st [1]. The total sum of the oscillator strength should be equal to the number of electrons in CH 4 and the result gives 9.90 as shown in Table 2, only 1% less than 10. The photoionization quantum yields, h, of CH 4 are shown in Fig. 4 along with the results by other methods: the method with a double ionization chamber without windows / an Ar discharge lamp [55], the dipole simulation method [56], the method with a single ionization chamber without windows / discharge lamps [58], and the method with a double ionization chamber / discharge lamp [59]. Except for the result by Metzger and Cook [58] using a single ionization chamber without windows and that of Wainfan [59], the curves in Fig. 4 show almost the same tendencies. Our method using a double ionization chamber equipped with metallic thin film windows and SR as a continuous-wavelength light source provides an excellent means for the measureTable 2 Oscillator strength distribution in CH 4

Fig. 3. The photoabsorption cross sections (st ) of CH 4 as a function of the incident photon energy in Fig. 2 along with results by other groups, for which references are listed in the figure. ‘High resolution’ and ‘low resolution’ in Au et al. [54] are 48 meV and 1 eV, respectively.

Energy range (eV)

Sum of the oscillator strength

$ 23.8 10.95–23.8 8.55–10.95

5.15 a 4.47 b 0.28 c

Total

9.90

a

From Ref. [55]. Present result. c From Ref. [71]. b

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Fig. 4. The photoionization quantum yields (h ) of CH 4 as a function of the incident photon energy measured in this experiment with a bandpass of 0.1 nm, which corresponds to the energy width of 32 meV at the incident photon energy of 20 eV. Results by other groups are also shown, for which references are listed in the figure. The vertical ionization potentials of the ionic states involved are indicated by the vertical bars [53] along with the first adiabatic ionization potential by the arrow [60].

ments of absolute h values and thus we are able to observe a small but discernible deviation of h from unity around 22 eV (see also Fig. 15), which shows the existence of the superexcited states built on the (2a 1 )21 ion state. They are discussed in Section 3.1.2 in more detail. This deviation from unity in the energy range of the inner valence orbitals associated with C 2s orbitals is a remarkable feature for not only CH 4 but also for C 2 H 6 and C 3 H 8 as shown later.

3.1.2. Superexcited Rydberg states As clearly seen in the sd curve of Fig. 2 there exist superexcited states in the range of 13–16 eV and in the range of 19–24 eV. We note that some of them show clear vibrational progressions, which seems to indicate that they are superexcited Rydberg states. There is also a possibility that non-Rydberg superexcited states play a role in these energy ranges. Expanded views in the energy ranges of 10–14 eV and 19–24 eV in Fig. 2 are displayed in Figs. 5 and 6, respectively, in order to show the vibrational progressions in detail. In Fig. 5 the vibrational progressions with spacing

231

Fig. 5. An expanded view of Fig. 2 partly in the incident photon energy range of 10–14 eV to show clearly the vibrational progressions of the Rydberg states converging to the (1t 2 )21 ion state. The resolution of the incident photon energy at 12 eV is 12 meV (see also Section 2). The vertical ionization potential of the (1t 2 )21 is indicated by the vertical bar [53] along with the first adiabatic ionization potential by the arrow [60].

of about 0.10–0.15 eV (800–1200 cm 21 ) are observed superimposed on the broad continuum. A high resolution He I photoelectron spectrum shows a broad (1t 2 )21 peak accompanied by four vibrational progressions with the spacing of 0.1–0.2 eV [60].

Fig. 6. An expanded view of Fig. 2 partly in the incident photon energy range of 19–24 eV to show clearly the vibrational progressions of the Rydberg states, (2a 1 )21 (npt 2 ) 1 T 2 (n 5 3, 4), which is based on the assignment by Mitsuke et al. [12]. The resolution of the incident photon energy at 20 eV is 32 meV (see also Section 2). The vertical dashed lines indicate the energy of the peaks in the sd curve (see the text). The vertical ionization potential of the (2a 1 )21 is indicated by the vertical bar [53].

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The vibrational progressions observed in the present work are hence assigned to the Rydberg states 21 converging to the (1t 2 ) ion state. The same progressions were partially resolved in a st curve [54] and an electron energy-loss spectrum [61]. The vibrational progressions seem not to be observed in the si curve, indicating that the superexcited Rydberg states may decay mainly through neutral dissociation rather than autoionization. In Fig. 6 the vibrational progressions are again observed in st , si , and sd below the ionization potential of the (2a 1 )21 ion state, and thus they are assigned to the superexcited Rydberg states converging to the (2a 1 )21 ion state. These superexcited Rydberg states were also observed in the excitation spectra for the formation of H 2 [12,13], threshold photoelectrons [62,63], visible photons [64], and VUV photons [65], and were well assigned to the (2a 1 )21 (npt 2 ) 1 T 2 states (n 5 3, 4) with the progression of the totally symmetric stretching mode (n1 ) [12], which we follow as in Fig. 6. The peak energies in the sd curve are in agreement with those in the excitation spectra mentioned above except for the formation of visible and VUV photons [64,65]. The reason for this disagreement is not clear at present. It is also remarkable in Fig. 6 that the energies of the peaks in the sd curve seem to be in good agreement with the resonance energies in the Fano profiles seen in the st and si curves. Each vibrational level of the superexcited Rydberg states, (2a 1 )21 (npt 2 ) 1 T 2 , in the sd curve displays Fano profiles with profile indices q having values close to zero in the st and si curves, showing the interesting features in the competition between autoionization and neutral dissociation. It is noted that the agreement between the energies mentioned above is not influenced by the uncertainties in the calibration of the wavelength of the incident photon, since the st , si and sd values in the present experiment were measured in the same wavelength-scan of the incident photon.

3.2. C2 H6 (ethane) The electron configuration of the ground state C 2 H 6 in D3d symmetry is [53]:

KK (2a 1g )2 (2a 2u )2 (1e u )4 (3a 1g )2 (1e g )4 . #%%%"!%%%$ #%%%%"!%%%%$ inner valence

outer valence

All the ionization potentials of the valence electrons are within the incident photon energy range involved (see Fig. 1). In the previous paper [46] we reported the absolute values of the st , si , sd , and h of C 2 H 6 in the incident photon energy range of 10–24 eV, which are shown in Figs. 7–9 for comparison with results by other groups [54,57,58,66,67]. As shown in Figs. 8 and 9, the st and h curves of C 2 H 6 measured by other groups show some significant differences from those obtained by our group. It is again concluded, however, that the use of metallic thin film windows with a double ionization chamber in our experiments contributes to a reliable measurement of st and, in particular, h as described in Section 2. The st curves by Metzger and Cook [58] and Lee et al. [57] deviate from the others. A large difference between the h curve of our group and those of Schoen [67] and Metzger and Cook [58] is attributed to the fact that they used an ionization chamber without windows. The h curve by Au et al. [66] is shifted to the lower energy side of the first

Fig. 7. The photoabsorption (st ), photoionization (si ), and neutral-dissociation (sd ) cross sections of C 2 H 6 as a function of the incident photon energy measured by our group [46] with a bandpass of 0.09 nm, which corresponds to the energy width of 29 meV at the incident photon energy of 20 eV. The vertical ionization potentials of the ionic states involved are also indicated by the vertical bars [53].

K. Kameta et al. / Journal of Electron Spectroscopy and Related Phenomena 123 (2002) 225 – 238

Fig. 8. The photoabsorption cross sections (st ) of C 2 H 6 as a function of the incident photon energy in Fig. 7 along with results by other groups, for which references are listed in the figure. ‘High resolution’ and ‘low resolution’ in Au et al. [54] are 48 meV and 1 eV, respectively.

adiabatic ionization potential as seen in Fig. 9, which seems to be attributed to their inaccurate energy calibration, and poor energy resolution of approxi-

233

mately 1 eV. It should be noted that they assumed h 51 around 20 eV, and thus the deviation from unity in this inner-valence range observed by our group [46] vanishes in their h curve. In the energy range of 18.5–20.5 eV, the vibrational progressions, clearly revealed in our experiment as shown in Fig. 7, do not appear in the st curve by Au et al. [54] probably because their energy resolution, 48 meV, is poorer than that in Fig. 7, 29 meV at 20 eV incident photon energy, and their data points are scattered. These vibrational progressions are assigned to the superexcited Rydberg states converging to the (2a 2u )21 ion state, which contribute to a small but discernible deviation of h from unity as shown in Fig. 9.

3.3. C3 H8 ( propane) The electron configuration of the ground state C 3 H 8 in C2v symmetry is [53]: KKK (3a 1 )2 (2b 2 )2 (4a 1 )2 #%%%%"!%%%%$ inner valence 2

2

2

2

2

2

2

(1b 1 ) (5a 1 ) (3b 2 ) (1a 2 ) (4b 2 ) (6a 1 ) (2b 1 ) . #%%%%%%%%%%"!%%%%%%%%%%$ outer valence

Fig. 9. The photoionization quantum yields (h ) of C 2 H 6 as a function of the incident photon energy measured by our group [46] with a bandpass of 0.09 nm, which corresponds to the energy width of 29 meV at 20 eV incident photon energy. Results by other groups are also shown, for which references are listed in the figure. ‘Low resolution’ in Au et al. [66] is 1 eV. The vertical ionization potentials of the ionic states involved are indicated by the vertical bars along with the first adiabatic ionization potential by the arrow [53].

All the ionization potentials of the valence electrons are within the incident photon energy range involved (see Fig. 1). In the previous paper [46], we reported the absolute values of the st , si , sd , and h of C 3 H 8 in the incident photon energy range of 10–24 eV, which are shown in Figs. 10–12 for comparison with results by other groups [54,66,67]. As with the case of C 2 H 6 , the st curves of C 3 H 8 measured by other groups are generally in good agreement with those by our group except for those by Schoen [67] (Fig. 11). The h curve by Au et al. [66] is again shifted to the lower energy side than the first adiabatic ionization potential (see Fig. 12) for the same reason as in C 2 H 6 . A large difference between the st and h curves of our group and those of Schoen [67] is again attributed to the use of an ionization chamber without windows. The sd curve around 20 eV in Fig. 10 and the corresponding deviation from unity in the h curve in Fig. 12 indicate that there exist superexcited states around 19 and 22 eV. We have assigned them to the superexcited states built on the (2b 2 )21 ion state and

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Fig. 10. The photoabsorption (st ), photoionization (si ), and neutral-dissociation (sd ) cross sections of C 3 H 8 as a function of the incident photon energy measured by our group [46] with a bandpass of 0.09 nm, which corresponds to the energy width of 29 meV at the incident photon energy of 20 eV. The vertical ionization potentials of the ionic states involved are also indicated by the vertical bars [53].

(3a 1 )21 ion state, respectively. Weak vibrational progressions are also seen around 18–19 eV in the st and si curves, which have been assigned to the superexcited Rydberg states converging to the (4a 1 )21 ion state.

Fig. 12. The photoionization quantum yields (h ) of C 3 H 8 as a function of the incident photon energy measured by our group [46] with a bandpass of 0.09 nm, which corresponds to the energy width of 29 meV at the incident photon energy of 20 eV. Results by other groups are also shown, for which references are listed in the figure. ‘Low resolution’ in Au et al. [66] is 1 eV. The vertical ionization potentials of the ionic states involved are indicated by the vertical bars along with the first adiabatic ionization potential by the arrow [53].

3.4. n-C4 H10 (normal butane) The electron configuration of the ground state n-C 4 H 10 in C2h symmetry is [53]: KKKK (3a g )2 (3b u )2 (4a g )2 (4b u )2 #%%%%%"!%%%%%$ inner valence 2

2

(1a u ) (5a g ) (1b g )2 (5b u )2 (6b u )2 (6a g )2 (2a u )2 (2b g )2 (7a g )2 . #%%%%%%%%%%%"!%%%%%%%%%%%$ outer valence

Fig. 11. The photoabsorption cross sections (st ) of C 3 H 8 as a function of the incident photon energy in Fig. 10 along with results by other groups, for which references are listed in the figure. ‘High resolution’ and ‘low resolution’ in Au et al. [54] are 48 meV and 1 eV, respectively.

Most of the ionization potentials of the valence electrons are within the present incident photon energy range of 13–24 eV. Fig. 13 shows the absolute st values of n-C 4 H 10 as a function of the incident photon energy obtained in the present experiment along with those by other groups [54,57,67,68]. The values of st by Schoen [67] and by Au et al. [54] agree with the present ones. The results by Koch and Skibowski [68] are much smaller than the others, probably because of a lack of window on their gas cell in the photoattenuation method.

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235

function of the photon energy [1]. In the present work, a more detailed overview is presented of these quantities for C 1 –C 4 normal alkanes as a function of the photon energy and of the number of C atoms in the alkane molecule.

4.1. Photoabsorption cross sections of normal alkanes

Fig. 13. The photoabsorption cross sections (st ) of n-C 4 H 10 as a function of the incident photon energy measured in this experiment with a bandpass of 0.36 nm, which corresponds to the energy width of 116 meV at the incident photon energy of 20 eV. Results by other groups are also shown, for which references are listed in the figure. ‘High resolution’ and ‘low resolution’ in Au et al. [54] are 48 meV and 1 eV, respectively.

In Fig. 13, there are no noticeable vibrational progressions in the range of the inner valence orbitals while they are seen in CH 4 , C 2 H 6 , and C 3 H 8 as described above, which is at least partly due to the lower resolution of the incident photon energy, i.e., 116 meV for n-C 4 H 10 , 32 meV for CH 4 , 29 meV for C 2 H 6 and C 3 H 8 at 20 eV incident photon energy (see also Section 2). An n-C 4 H 10 molecule has two stable conformations, i.e., gauche and anti. This means the photoabsorption cross sections we have measured are mean values over the gauche and anti conformers of n-C 4 H 10 under room temperature. Therefore, temperature effects on the st are an interesting subject in future in terms of the change of the number density ratio between the different conformers.

In this section we compare the photoabsorption cross sections of the C 1 –C 4 normal alkanes in order to obtain an overview of the interaction between VUV photons and valence electrons in normal alkanes. Fig. 14 shows the photoabsorption cross sections, st , of CH 4 , C 2 H 6 , C 3 H 8 , and n-C 4 H 10 measured by our group, and the following features can be noted. (1) Each cross section has a maximum around 13–16 eV, and the observed maxima shift to the higher energies with increasing number of carbon atoms, i.e., from CH 4 to n-C 4 H 10 , while the first ionization potentials shift to the lower energies as shown in Fig. 1. This shift of the maxima seems to be consistent with the trends in the ionization potentials of the deepest outer valence orbitals of each molecule. (2) In the lower energy range below the peak maximum the st curves show some undulations, which are

4. Photoabsorption and photoionization of normal alkanes—an overview An overview has been presented more comprehensively of the photoabsorption cross sections, i.e., the oscillator strength distributions, and the photoionization quantum yields of polyatomic molecules, providing several features in general of these values as a

Fig. 14. The photoabsorption cross sections (st ) of CH 4 , C 2 H 6 , C 3 H 8 , and n-C 4 H 10 as a function of the incident photon energy. They are cited from Figs. 2, 7, 10, and 13, respectively. The energy resolution of the incident photon energy at 20 eV is as follows: 32 meV for CH 4 , 29 meV for C 2 H 6 , 29 meV for C 3 H 8 , and 116 meV for n-C 4 H 10 .

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attributed to either excitation, superexcitation, or ionization of the outer valence electrons. (3) Around 18–23 eV, vibrational progressions are clearly observed for CH 4 , C 2 H 6 , and C 3 H 8 , which have been assigned to the superexcited Rydberg states converging to the (2a 1 )21 , (2a 2u )21 , and (4a 1 )21 ion states, respectively. The superexcited Rydberg states move to lower energies with increase in the number of carbon atoms corresponding to the order of the ionization potentials of the 2a 1 electron in CH 4 , 2a 2u electron in C 2 H 6 , and 4a 1 electron in C 3 H 8 as shown in Fig. 1. It is difficult with existing theoretical knowledge to explain the gross features in the shapes of the st curves in Fig. 14. In this context it should be noted that Nakatsukasa and Yabana [69] have recently calculated the absolute values of the st as a function of the energy up to 40 eV for SiH 4 (silane), C 2 H 2 (acetylene) and C 2 H 4 (ethylene) based on the timedependent local density approximation, and found good agreement with the results by the real-photon [41,45,70] and virtual-photon experiments [28,36,37].

4.2. Photoionization quantum yields of normal alkanes The photoionization quantum yields, h, of CH 4 , C 2 H 6 , and C 3 H 8 measured by our group are compared with each other in Fig. 15. The photon energies are considered in two ranges as follows in terms of the behavior of the h curves as a function of the photon energy. (1) Firstly, the energy range of the outer valence electrons, where the h curves rise up gradually and then reach unity, and (2) secondly, the energy range of the inner valence electrons, i.e., higher than approximately 17 eV, where the h curves show small but discernible deviations from unity corresponding to the existence of the superexcited states as mentioned in Sections 3.1–3.3. The h curves rise up from the first adiabatic ionization potentials and then reach unity at photon energies slightly higher than the ionization potentials of the deepest outer-valence electrons. The energy differences between the onsets of the h curves and the photon energies where the h curves reach unity are approximately 3 eV for CH 4 , 5 eV for C 2 H 6 , and 6 eV for C 3 H 8 , reflecting the difference in the

Fig. 15. The photoionization quantum yields (h ) of CH 4 , C 2 H 6 , and C 3 H 8 as a function of the incident photon energy. They are cited from Figs. 4, 9 and 12, respectively. The energy resolution of the incident photon energy at 20 eV is as follows: 32 meV for CH 4 , 29 meV for C 2 H 6 , and 29 meV for C 3 H 8 . The vertical ionization potentials of the ionic states involved are indicated by the vertical bars for each molecule [53].

ionization potentials between the shallowest and the deepest outer-valence orbitals shown in Figs. 1 and 15.

5. Conclusions In the st curves of CH 4 , C 2 H 6 , C 3 H 8 , and nC 4 H 10 the results of our SR experiments using real photons are in quantitatively good agreement with those of dipole-simulation experiments using virtual photons. In the case of the h curves of these alkanes the discrepancies between the real- and virtualphoton experiments are larger, which seems to be

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due to the low energy resolution (1 eV) and larger uncertainty in the energy scale (61 eV) of the virtual-photon experiments. From our results for the st and h curves the absolute magnitudes of these values have been overviewed as a function of the number of C atoms in these alkanes. A small but discernible deviation of h from unity around 20 eV for each of these (C 1 – C 3 ) alkanes has been observed in the range of the inner valence electrons. This deviation is the evidence for the existence of the superexcited states with the excitation of an inner valence electron. The present experimental method using a double ionization chamber equipped with metallic thin film windows and SR as a continuous-wavelength light source is a powerful method for absolute measurements of st , si , sd and h. A normal alkane molecule cannot easily be ionized in the range of the outer valence electrons, i.e., the range lower than approximately 17 eV. The accurate values of st and h enable us to obtain the si and sd values which are more sensitive to the existence of the superexcited states than st . We have found that each vibrational level of the superexcited Rydberg states of CH 4 , (2a 1 )21 (npt 2 ) 1 T 2 (n 5 3, 4), in the sd curve shows Fano profiles in the st and si curves with profile indices q having values close to zero.

[4]

[5] [6]

[7] [8] [9]

[10]

[11]

[12] [13] [14] [15] [16] [17] [18] [19] [20]

Acknowledgements

[21]

This work has been performed under the approval of the Photon Factory Program Advisory Committee for proposal No. 94G-375. We are grateful to the staff of the Photon Factory. The support of the Ministry of Education, Science, Sports and Culture, Japan in providing a Grant-in-Aid for Specially Promoted Research, No. 08102005, is gratefully acknowledged.

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