Valence–shell electron energy-loss spectra of formic acid and acetic acid

Journal of Electron Spectroscopy and Related Phenomena 106 (2000) 29–35 www.elsevier.nl / locate / elspec

Valence–shell electron energy-loss spectra of formic acid and acetic acid ¨ Tayfun Ari*, M. Haluk Guven Middle East Technical University, Department of Physics, 06531 Ankara, Turkey Received 1 March 1999; received in revised form 30 August 1999; accepted 31 August 1999

Abstract Gas phase optical absorption spectra of formic acid and acetic acid in the energy ranges 5.0–11.3 and 4.4–11.3 eV, respectively, have been previously recorded. It is generally agreed that of the four successive bands observed in the spectra of these compounds, the first corresponds to n → p * transition. However, various assignments have been proposed for the following three bands. In the present study, electron energy-loss spectra of formic acid and acetic acid vapours at 70 eV impact energy and 08 scattering angle have been measured between 5–15 eV. The transition energies that have been observed coincide with those measured by optical methods in the corresponding range. To identify the valence shell transitions among the spectral features observed, the semi-empirical HAM / 3 calculations have been performed. With the help of earlier studies on these and related compounds first and third bands are tentatively assigned as valence, whereas second and fourth bands are assigned as Rydberg.  2000 Elsevier Science B.V. All rights reserved. Keywords: Acetic acid; Formic acid; Gas phase optical absorption; Valence shell transition

1. Introduction Studies of valence shell electronic excitations of formic acid and acetic acid in the vapour phase include UV photo absorption measurements below first ionization potentials of these compounds [1]. Valence shell electron energy loss spectrum of formic acid has also been reported [2]. To our knowledge there is no published valence–shell electron energy loss spectrum of acetic acid. Electronic structure of carboxylic acids and their derivatives also has been the subject of PE measurements and theoretical calculations [3–5]. These studies indicate that the orbital localized at the oxygen atom of the carbonyl group and having a n o *Corresponding author. Fax: 190-312-210-1281.

character seems to be the highest lying occupied molecular orbital. The next is the p molecular orbital of the OCO framework. The vibrational structure analysis of the corresponding photoelectron bands show that both are essentially of nonbonding character. From these findings it is expected that lowest lying features in the UV spectra of carboxylic acids are due to valence shell transitions from n and p molecular orbitals. On the other hand, UV photo absorption measurements in solution and vapour phase indicate that Rydberg states lie close to the excited valence shell states in many cases, causing the complexity of the spectra. Various assignments have been proposed of the transitions observed. However, it is generally agreed that the lowest absorption band in the UV spectra of carboxylic acids is due to n o → p * transition.

0368-2048 / 00 / $ – see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S0368-2048( 99 )00084-5

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¨ / Journal of Electron Spectroscopy and Related Phenomena 106 (2000) 29 – 35 T. Ari, M. Haluk Guven

In the present study we undertook to perform an electron energy-loss study of formic and acetic acid vapours at 70 eV impact energy and 08 scattering angle.To help identify the valence shell transitions among the others semi-empirical density matrix HAM / 3 [6] calculations designed to work for valence shell excitations were also carried out. The method is based on the total energy expression of the density functional formalism [7]. After LCAO expansion of Kohn–Sham orbitals in terms of valence orbitals of the constituent atoms the total energy appears in the form where one and two-center contributions are separated. Shieldings which take care of one-center terms describing the internuclear attraction, electron–electron repulsion, and pair correlation energies are determined from electron affinity, photoelectron, and excitation studies on large number of atomic and molecular species. In the calculations of overlap integrals Slater-type orbitals with fixed orbital exponents are used. Application of variational procedure on the total energy then leads to Roothan’s equations. After solving the equations iteratively a set of values corresponding to the energies of occupied and unoccupied molecular orbitals are obtained. The excitation energy is the difference of the total energies of the initial and final states. To take care of the reorganization in excitation the transition state method [6,7] may be used. However, this is less important than for ionization since the number of electrons in the molecule does not change in the excitation. Since no self-repulsion included in the calculations it is then possible to obtain the excitation energies as the difference of the orbital energies [6]. Optimum geometries of formic and acetic acids found by AMI [8] method were used in the calculations.

2. Experimental The electron energy loss spectrometer used in the present experiment is similar in design to that of Simpson [9]. The whole instrument is in a vacuum system which is evacuated by a diffusion pump in tandem with a rotary pump. A base pressure of 10 26 Torr was achieved. Steady ambient magnetic fields in the laboratory environment were eliminated by

placing the apparatus inside three mutually perpendicular sets of Helmholtz coils. A 1808 hemispherical monochromator is employed as an energy dispersing component for the electrons extracted from thoriated tungsten hairpin filament. By means of a lens system with deflectors located at the exit of the monochromator a collimated beam of electrons with a narrow energy spread is focussed onto target molecules contained in a collision chamber. Electrons leaving the collision chamber pass through an acceleration lens system and enter into the analyser which is identical to the monochromator. Initially the analyser section is adjusted to allow only the electrons that have not lost energy to reach the detector. Subsequently,the accelerating voltage at the entrance to analyser is scanned so that the electrons which have lost a variable amount of energy in the course of collision with the target molecules are transmitted and collected at the channel electron multiplier. The resolution and the angular spread of the electron beam were 0.06 eV and 61.58, respectively, for 70 eV incident electron energy. A signal average of 15–20 scans were required to obtain a spectrum with a good signal-to-noise ratio. The energy scale of the spectra were calibrated against the well established 11.62 and 11.83 eV peaks in the energy loss spectrum of argon.

3. Results and discussion

3.1. Formic acid UV spectra of formic acid in the vapour phase in the ranges 8.0–9.7, 5.0–9.9, 6.2–8.3, and 6.9–11.3 eV have been reported by Price and Evans [10], Barnes and Simpson [11], Nagakura [12], and Bell et al. [13], respectively. Previous valence–shell electron energy loss study has been performed by Fridth [2] at an impact energy of 500 eV and a scattering angle of 08. Our spectrum is shown in Fig. 1. The experimental and calculated transition energies and their possible assignments are given in Table 1. From a comparison of observed and calculated positions and intensities the very weak absorption feature at about

¨ / Journal of Electron Spectroscopy and Related Phenomena 106 (2000) 29 – 35 T. Ari, M. Haluk Guven

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Fig. 1. Sample recorded electron energy-loss spectrum of formic acid vapour at an incident electron energy of 70 eV and a scattering angle of 08.

Table 1 Experimental and calculated transition energies (eV) of formic acid Possible assignments

This work Exp.

Ham / 3

no → p * n o →3s 4s p2 → p * n o →3p 4p

5.8 7.7 10.1 8.4 8.9 10.1

6.0

a

Taken Taken c Taken d Taken b

from from from from

Ref. Ref. Ref. Ref.

[17]. [13]. [15]. [14]

8.7

Optical studies

Other calculations

4.78–5.9 a 7.8 b 9.94 b 8.10 b 8.95 b 10.15 b

5.84 c , 6.9 d 8.14 c 10.00 c 9.64 c , 12.02 d 9.16 c 10.26 c

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¨ / Journal of Electron Spectroscopy and Related Phenomena 106 (2000) 29 – 35 T. Ari, M. Haluk Guven

5.8 eV in Fig. 1 is identified as n o → p * transition on which there is general agreement [1]. Above the 5.8 eV band we observe two strong energy loss bands at about 7.7 and 8.4 eV. From a comparative study of various carbonyl and carboxyl compounds Barnes and Simpson have tentatively assigned corresponding bands in the optical spectrum to n o9 → p *, where n 9o is the second lone pair orbital on the carbonyl group oxygen, and p2 → p *. Bell et al. suggested instead that 7.7 eV was p2 → p * and 8.4 eV maximum was the n53 member of an ns series of quantum defect d 50.85. However in the study of amides, carboxylic acids, and esters by Basch et al. [14] valence character of the band corresponding to 7.7 eV maximum has been refuted by condense phase spectrum of trifluoric acid. As has been known Rydberg transitions of whatever prominence in the gas phase simply do not appear in condensed phases. The HAM / 3 calculations we have performed predict very low intensities for the valence–shell transitions in the energy range presently studied except for p2 → p * transition which is calculated to occur at 8.7 eV. Therefore in agreement with the findings of Basch et al. [14] we consider that 8.4 eV maximum is due to the p2 → p * transition. Hence the 7.7 eV maximum is assigned as n o →3s of quantum defect d 51.1. Furthermore, a comparison of the intensities of 7.7 and 8.4 eV peaks in Fig. 1 with relative oscillator strengths of the n o →3s and p2 → p * transitions computed by Demoulin [15] supports these assignments. Bell et al. [13] have reported two other Rydberg series converging to the first ionization potential. The bands of both series show the same vibrational interval, n 50.186 eV, as the first band of the photoelectron spectrum which is due to the ionization of n o . The first series with quantum defect of 0.60, starts with a strong band at 8.95 eV and was assigned an np series. This is the series originally found by Price and Evans [10]. In the spectrum of Fig. 1 the intensity maximum at about 8.9 eV has the term value 2.56 eV and corresponding quantum defect of 0.7 with respect to the first ionization potential at 11.51 eV. A comparison of its relative intensity with the computed value by Demoulin [15] suggests that this can be assigned as n o →3p in agreement with Bell et al. [13]. The structure in this area may be due to the symmetry splitting of 3p

states. In the same region one might further find p2 →3s. However its computed oscillator strength is very low. We predict from the Rydberg formula that the higher order members of the n o → np series is to occur at 10.3 and 10.8 eV for n54 and n55. Therefore the part of the intensities of the features at about 10.1 and 10.6 eV in Fig. 1 may be attributed to these transitions respectively. The second series that Bell et al. [13] have reported starts with a band at 9.65 eV and has quantum defect, d 50.14. This was assigned an nd series. From the Rydberg formula n53 and 4 members of this series are predicted to occur at 9.85 and 10.57 eV. Hence, the part of the intensity of the strong feature at 10.1 eV and weaker feature at about 10.6 eV may be due to these transitions.

3.2. Acetic acid The optical absorption spectrum of acetic acid in the vapour phase in the ranges between 4.4–9.9, 6.2–8.3, and 4.8–11.3 eV have been recorded by Barnes and Simpson [11], Nagakura et al. [12], and Bell et al. [13], respectively, and in the vapour phase and in n-heptane solution by Platt et al. [16]. The spectrum of acetic acid obtained in the present study is shown in Fig. 2. Excitation energies found experimentally and evaluated by the use of HAM / 3 method are given in Table 2. Basic similarity of the spectra of carboxylic acids have been shown in the previous optical studies [1]. A comparison of spectra of Figs. 1 and 2 also appears to support these findings. From the observed and calculated excitation energies and intensities and previous optical studies it is almost certain that the very weak feature at about 5.8 eV in Fig. 2 is due to n o → p * transition located at 5.9 eV in the optical spectrum. We observe three maxima at about 7.1, 7.8, and 8.5 eV in Fig. 2. Corresponding bands in the optical spectrum have been interpreted by Barnes and Simpson as due to the n o9 → p *, n o →s*, and p2 → p * transitions, respectively. On the other hand, in the study of vapour phase and n-heptane solution spectra of acetic acid [16], the optical band corresponding 7.1 eV maximum in Fig. 2. appears to be of Rydberg character and has been assigned as n o →3s transition. The 8.5 eV maximum was ascribed to

¨ / Journal of Electron Spectroscopy and Related Phenomena 106 (2000) 29 – 35 T. Ari, M. Haluk Guven

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Fig. 2. Sample recorded electron energy-loss spectrum of acetic acid vapour at an incident electron energy of 70 eV and a scattering angle of 08.

n o →3p by Bell et al. [13]. The assignments of Bell et al. [13] and Platt et al. [16] agree with the conclusion of study by Basch et al. [14] that first

Table 2 Experimental and calculated transition energies (eV) of acetic acid Optical studies a

Possible assignments

This Work Exp.

Ham / 3

no → p * n o →3s 4s p2 → p * n o →3p

5.8 7.1 9.3 7.8 8.5

6.3

a

Taken from Ref. [11].

8.5

5.9 7.21 9.0–9.3 7.8 8.16–8.86

four bands of carboxylic acids are n o → p *, n o →3s Rydberg, p2 → p *, and n o →3p Rydberg transitions. From the present HAM / 3 calculations, it appears that the spectrum of Fig. 2 is dominated by Rydberg transitions as in the case of formic acid. The excitation energies of n o → p * and p2 → p * transitions obtained from HAM / 3 are 6.3 and 8.5 eV, respectively. According to the assignments above calculated energies are higher by about 0.5 eV than the experimental values. At higher energies the optical absorption spectrum of acetic acid shows a number of diffuse bands starting at 9.0, 9.8, and 10.3 eV [11]. According to Bell et al. [13] they are probably of Rydberg character. No convergence limit has been established

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¨ / Journal of Electron Spectroscopy and Related Phenomena 106 (2000) 29 – 35 T. Ari, M. Haluk Guven

for these transitions. Referring to Fig. 2, the energy loss feature maximizing at about 9.3 eV may be correlated with the optical band starting at 9.0 eV, and. the broad feature maximizing at 10.3 eV may be partly accounted for by the transition n o →4s. From the quantum defect of d 51.1 it is predicted to occur at 9.3 eV. Part of the intensity of the latter feature at about 10.3 eV may be due to the higher order members of ns and np series converging to the first ionization potential at 10.87 eV.

and 6.6 eV, respectively. On the other hand, in accordance with the present assignments, the term values are observed to be 5.7, 4.1, and 5.1, 4.3 eV for n o → p * and the p2 → p * transitions of formic acid and acetic acid, respectively. The term values are lowered as expected. In this regard, a larger electron interaction between the valence hole and the excited electron may be a contributing factor to the larger decrease observed in the term values for the p2 → p * transitions than that of the n o → p * transitions [20].

4. Concluding remarks Acknowledgements Using an electron spectrometer valence shell excitations of formic acid and acetic acid have been recorded. Previous spectroscopic informations and HAM / 3 calculations have provided a base for our assignment of these spectra. It appears that our Rydberg assignments are supported by the ISEELS (inner-shell electron energy loss spectroscopy) studies on these molecules [18,19]. It has been demonstrated earlier that the term values are transferable between the ISEELS and VSEELS (valence– shell electron energy loss spectroscopy) for Rydberg transitions [20]. Such constancy of term values for transitions from different orbitals to a particular Rydberg orbital is explained in terms of Rydberg orbitals are being large and hence will see the rest of the molecule as a singly positively charged center regardless of the location of the vacancy in the molecule. According to the present assignments of formic acid and acetic acid spectra, the term values for 3s and 3p Rydberg transitions are found to be 3.8, 2.6 and 3.8, 2.4 eV, respectively. The average term values of corresponding transitions are 3.8, 2.5 and 4.4, 2.5 eV, respectively, in the ISEELS spectra. A comparison of these values shows that they are in close agreement except for the 3s transition in acetic acid. It has also been observed that in general the term values for transitions to valence unoccupied orbitals in VSEELS spectra are lower than corresponding ISEELS transitions [20]. According to the ISEELS studies, the average term values for the transitions to the p * orbitals of formic acid and acetic acid are 6.7

The authors thank Professor J.B. Hasted for providing facilities at Birkbeck College, University of London, to enable one of us to carry out the experimental work

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