Electronic state spectroscopy by high-resolution vacuum ultraviolet photoabsorption, He(I) photoelectron spectroscopy and ab initio calculations of ethyl acetate

The high-resolution vacuum ultraviolet photoabsorption spectrum of ethyl acetate, C4H8O2, is presented over the energy range 4.5−10.7 eV (275.5−116.0 nm). Valence and Rydberg transitions and their associated vibronic series observed in the photoabsorption spectrum, have been assigned in accordance with new ab initio calculations of the vertical excitation energies and oscillator strengths. Also, the photoabsorption cross sections have been used to calculate the photolysis lifetime of this ester in the upper stratosphere (20−50 km). Calculations have also been carried out to determine the ionisation energies and fine structure of the lowest ionic state of ethyl acetate and are compared with a newly recorded photoelectron spectrum (from 9.5 to 16.7 eV). Vibrational structure is observed in the first photoelectron band of this molecule for the first time.


Introduction
The atmospheric degradation of commonly used volatile organic compounds (VOC) has been of great interest due to their contribution to the green-house effect and acidrain chemistry. Many such compounds are also known to be formed in the atmosphere due to radical reactions with other compounds that are being released into the atmosphere due to industrial activity, thus it is important to learn more about their chemistry through investigations of the electronic structure of such chemicals.
Esters are one of the most important classes of oxygenated VOCs that are both of industrial and astrochemical interest. Their prominent biogenic source is through Contribution to the Topical Issue "Low-Energy Interactions related to Atmospheric and Extreme Conditions", edited by S. Ptasinska, M. Smialek-Telega, A. Milosavljevic, B. Sivaraman. a e-mail: smialek@pg.gda.pl their appearance in some fruits, nonetheless, the main source of release to the atmosphere is due to industrial activities. They are mainly used as solvents, fumigants, flavouring and scenting agents. They can also be produced in the troposphere due to hydroxyl radical attack on ethers. It has also been shown that small esters are present in the interstellar space [1]. The significance of ethyl acetate is in its wide use in industry as a solvent, as well as in production of printing ink, glue or paint [2]. Its presence was also detected in the atmosphere of forests in Central Europe due to biogenic sources emissions [3].
Here we report a high resolution VUV photoabsorption spectrum with absolute cross sections of ethyl acetate measured for the first time, accompanied by a photoelectron spectrum (PES) and ab initio calculations on the vertical excitation energies and oscillator strengths of the electronic transitions. Measurement and analysis of the photoelectron spectrum aided the assignment of possible Rydberg transitions in the VUV photoabsorption spectrum. Moreover, for the first time it was possible to resolve and assign vibronic structure in the first ionic band. The only available studies of this molecule concern its first ionisation energy that was obtained either through electron impact mass spectrometry [4][5][6] or conventional photoelectron measurements [7][8][9] and PIPECO spectroscopy [10]. The absolute photoabsorption cross sections of ethyl acetate presented here are needed for modeling studies of the Earth's atmosphere and radiationinduced chemistry of esters, and, together with the results on previously analysed members of this class: methyl formate [11], ethyl formate [12] and isobutyl formate [13], will help to understand the chemistry of esters and their electronic state spectroscopies.

Structure of ethyl acetate
Among all the possible conformers, trans-trans and transgauche are reported to be the most abundant for the ethyl acetate molecule, with the trans-trans being the most stable at room temperature [14]. In the most stable form in the gas phase, ethyl acetate, C 4 H 8 O 2 , has C s symmetry ( Fig. 1). In our previous studies on the related compounds of ethyl formate [12] and isobutyl formate [13] it was shown that a proper assignment of the spectroscopic features is obtained using calculations on a single conformer. Therefore, in the present study theoretical computations are only performed on the trans-trans conformer. According to our calculations, the electron configuration of theX 1 A ground state is as follows: (a) core orbitals The highest occupied molecular orbital (HOMO, 19a ) in the neutral ground state is localized predominantly on the terminal oxygen in-plane lone pair (n O1 ). The second highest occupied molecular orbital (HOMO-1, 5a ) shows a main contribution from the oxygen out-of-plane lone pair (n O2 ). The orbital 4a (HOMO-2) corresponds mostly to the bonding π(C=O). The lowest unoccupied molecular orbital (LUMO) is mainly of π* antibonding character and it is localized on the C=O bond.

Ethyl acetate sample
The liquid sample used both in the VUV photoabsorption measurements and the PES experiment was purchased from Sigma-Aldrich, with a purity of 99.8%. The sample was degassed by six freeze-pump-thaw cycles, with no further purification of the sample.

VUV photoabsorption
The high-resolution VUV photoabsorption spectrum of ethyl acetate (Fig. 2) was measured at the UV1 beam line using the ASTRID synchrotron facility at Aarhus University, Denmark. The experimental set up has been described in detail previously [15]. Briefly, monochromatised synchrotron radiation passes through a static gas sample and a photomultiplier is used to measure the transmitted light intensity. The incident wavelength is selected using a toroidal dispersion grating with 2000 lines/mm providing a resolution of ∼0.075 nm. The minimum and maximum wavelengths between which scans are performed, 115−320 nm (10.8−3.9 eV), are determined by the cutoffs of the windows of the gas cell (LiF entrance and MgF 2 exit) and the grating respectively. The sample pressure is measured using a capacitance manometer (Baratron). In order to ensure that the data were free of any saturation effects [16,17], the cross section was measured at an appropriate pressure in the range 0.03−1.3 mbar, with typical attenuations <40%. In doing so, we avoided both absorption flattening (when the transmitted intensity is very low), and poorly defined (and hence noisy) spectra when the transmitted intensity is high but close to the incident intensity. The synchrotron beam current is monitored throughout the collection of each spectrum and a background scan is recorded for an where I t is the radiation intensity transmitted through the gas sample, I 0 is that through the evacuated cell, n is the molecular number density of the sample gas, σ is the absolute photoabsorption cross section, and x is the absorption path length (15.5 cm). The accuracy of the cross section is estimated to be better than ±5%. Only for low values of the absorption (I 0 ≈ I t ), the uncertainty increases as a percentage of the measured cross section. Specifically, we estimate that the lowest cross section peak of ethyl acetate (about 0.2 Mb near 5.9 eV) to have an uncertainty lower than 15%, which still amounts to only 0.03 Mb.

Photoelectron spectroscopy
The He(I) (21.22 eV) photoelectron spectrum of ethyl acetate was measured at the Université de Liège, Belgium and is shown in Figure 3. The apparatus has also been described previously in detail [18]. Briefly, it consists of a 180 • hemispherical electrostatic analyser with a mean radius of 5 cm. The analyser is used in the constant pass energy mode. The incident photons are produced by a d.c. discharge in a two-stage differentially pumped lamp. The energy scale is calibrated using the X 2 Σ + g , ν = 0 and A 2 Π u , ν = 0 peaks of N + 2 , rounded to three decimal places [19,20]. The resolution of the present spectrum is 55 meV and the accuracy of the energy scale is estimated to be ±2 meV. The photoelectron spectrum presented in this paper is the sum of 70 individual spectra. This procedure allows obtaining a good signal-tonoise ratio while keeping the pressure in the spectrometer low (<5 × 10 −6 mbar), thus minimizing the occurrence of dimers. For analysis purposes, the spectrum was also deconvoluted according to the van Cittert procedure [21], applied to the ratio method [22]. This technique was only applied in order to bring out some features predicted by the calculations that were hidden in the raw data and should be applied with some care, as described previously [23].

Computational methods
The ground state geometry, harmonic vibrational frequencies, and normal coordinates of the neutral singlet state (S 0 ) and ionic doublet state (D 0 ) of ethyl acetate were obtained with the Gaussian 09 program [43] by means of second-order Møller-Plesset (MP2) calculations in association with the aug-cc-pVTZ basis set. The ionic state was described by open-shell unrestricted calculations. The first ionisation energies (IE) were computed from the energy difference between the neutral and ionic ground states. The vertical IE was calculated at the ground state geometry of the neutral compound and the adiabatic IE was evaluated using the optimized geometries of the neutral and ionic ground states. Additionally, the first and second ionisation energies were calculated with the coupled-cluster singles and doubles (CCSD), and the coupled-cluster singles, doubles and perturbative triples (CCSD(T)) methods, employing the optimized geometries at the MP2/augcc-pVTZ level of approximation. Furthermore, the zero point vibrational energy (ZPVE) corrections to the adiabatic IEs was determined from the MP2/aug-cc-pVTZ harmonic vibrational frequencies. This correction was also applied to the values calculated with the CCSD and CCSD(T) methods. Higher IEs were obtained with the Partial Third Order (P3) and Outer Valence Green's Function (OVGF) propagator methods using the aug-cc-pVTZ basis set and the MP2/aug-cc-pVTZ geometry. The Franck-Condon (FC) factors, associated with the first photoelectron band of ethyl acetate, were calculated using recursive relations and include Duschinsky rotation effects [24][25][26][27]. These calculations made use of the ground state geometries, harmonic frequencies and normal coordinates calculated with the MP2/aug-cc-pVTZ method for the neutral (S 0 state) and ionic (D 0 state) forms of ethyl acetate. Next, the vertical excitation energies, oscillator strengths and electronic radial spatial extents of ethyl acetate were calculated with the equation of motion coupled cluster method restricted to single and double excitations (EOM-CCSD) using the MP2/aug-cc-pVTZ geometry. To provide a better description of the Rydberg states, a set of diffuse functions (6s, 6p, 4d), taken from Kaufmann et al. [28] and localized on the central oxygen atom (O2), was added to the aug-cc-pVTZ basis set (named aug-cc-pVTZ + R). These calculations were performed using the MOLPRO programme, ver. 2012.1 [29].

He(I) photoelectron spectrum
The He(I) photoelectron spectrum of ethyl acetate over the energy range 9.5−16.7 eV is shown in Figure 3.  [11][12][13]. The IE value obtained for the 2 A (5a −1 ) state, yielding 10.801 eV, is also comparable with that obtained previously, 10.99 eV [8]. It is worth noticing that the order of states 4a and 18a is inverted in the calculations with respect to the determined electron configuration of the molecule in its ground state. In Figure 4, a low energy part of the photoelectron spectrum with resolved 19a −1 state (top) is compared with the calculated spectrum obtained from the Franck-Condon factors (bottom). Since the calculated spectrum revealed possible vibronic structure, the experimental spectrum has been deconvoluted using Gauss profiles and is shown in the middle part of the Figure 4. The vibrational transitions calculated for ethyl acetate are shown in Table 2, together with corresponding FC factors. From this analysis it was possible to assign the main vibronic structure visible in the 19a −1 state to the ν 28 mode that designates C−O and C=O stretches. This structure is then accompanied by combination bands involving mainly the ν 7 mode that, according to our calculations, corresponds to OCC deformation combined with CO stretch and CH 3 rocking and can be labelled as the OCC out-ofplane bend [31]. Table 2 and Figure 4 show that mode ν 6 and to a lesser extent mode ν 8 also contribute to the vibronic structure. The energy positions of the vibrational features resolved in the first state in the photoelectron spectrum are presented in Table 3, and are assigned to progressions of modes ν 28 and ν 7 .

Rydberg series
The photoabsorption spectrum above 6.5 eV shows a number of superimposed structures extending to the lowest ionisation energy. The proposed Rydberg transitions, labeled in Figures 5 and 6 and presented in Tables 4 and 5, originate mainly from the HOMO (19a ) according to our calculations. The positions of the peaks, E n , have been predicted using the well-known Rydberg formula: where E i is the ionisation energy, R is the Rydberg constant 13.61 eV, n is the principal quantum number of the Rydberg orbital of energy E n , and δ is the quantum defect. Quantum defects are expected to be in range of 1.0−0.9, 0.6−0.3 and <0.25 for ns, np and nd series, respectively. The ionisation energy used in this assessment was the vertical value for 19a −1 , i.e. 10.226 eV. In the measured experimental spectrum, due to a massive overlap of electronic states that occurs already at low energy, it was not possible to assign the transitions  unambiguously, thus the higher members of all series should be treated rather as tentative assignments that were made based on the value of the quantum defect. The feature at 7.634 eV was assigned to the (3sσ ← n O1 , 19a ) Rydberg transition with a quantum defect of 0.71 (Tab. 4) and is accompanied by fine structure, discussed in Section 5.3. The calculated value of 7.343 eV is in a good agreement with the one found experimentally. The higher members of these Rydberg series are proposed to extend to n = 4. State n = 5 was assigned purely on the value of the quantum defect.
The first members of npσ and npπ series were found at 8.000 eV and 8.195 eV with quantum defects of 0.53 and 0.41, respectively (Tab. 4). Calculated energies for these transitions (Tab. 5), yielding 8.132 eV and 8.119 eV, agree with experimental values reasonably well, when the possible valence character of these transitions is also accounted for. First members of both np series are also associated with vibronic structure, as shown in Figure 5. The higher members of these Rydberg series were assigned tentatively, based on the value of the quantum defect. The features at 8.372 eV and 8.574 eV were assigned

Valence states and transitions of ethyl acetate
Calculated vertical excitation energies of the first thirty states of ethyl acetate, shown in Table 5, allowed assignment of the absorption band centred at 5.904 eV to (π*(C=O) ← n O1 19a ) transition and the one centred at 8.394 eV to the (π*(C=O) ← n O2 5a ) transition (Figs. 2 and 6). The first band, shown in Figure 2, with a maximum cross section of 0. 19 Mb, has been identified as the transition from the terminal oxygen in-plane lone pair (n O1 ) to the first π antibonding molecular orbital (π*(C=O) ← n O1 19a ) (1 1 A ← 1 1 A ). The calculated value of the oscillator strength, 1 × 10 −3 , is comparable with the values obtained for the previously analysed esters [11][12][13]. The position of the maximum cross section of this band also agrees well with the value given by others, obtained from the spectrum of the liquid sample, i.e. 5.904 eV [32]. No vibronic structure has been resolved on this band.
It was possible to assign some vibrational progressions in the 6.5−10.0 eV range over the bands identified as pure Rydberg transitions and the details of these assignments are shown in Figures 5 and 6, and are also listed in Table 6. Pure Rydberg transitions of high oscillator strengths from this region were discussed in Section 5.2 on the Rydberg series. The most pronounced series over the structure centred at 7.634 eV, from the 3sσ ←19a transition, is proposed to be due to the ν 28 mode (C−O and C=O stretches) with an average spacing of 0.174 eV, and with two quanta of the ν 7 mode (OCC deformation combined with CO stretch and CH 3 rocking), spaced on average by 0.054 eV. Another vibration, associated with the (3pπ ←19a ) transition is due to the ν 7 mode with an average spacing of 0.056 eV.
At 8.394 eV, a (π*(C=O) ← n O1 5a )(7 1 A ←1 1 A ) transition was assigned, with a maximum cross section of 13.19 Mb. According to our calculations this is one of the most pronounced features in the spectrum, with a highest value of the oscillator strength of 0.136.
In the higher energy range, where we see a continuous overlap of Rydberg transitions, it was also possible to distinguish two quanta of ν 28 mode, spaced by 0.180 eV, each coupled with one quantum of the ν 7 mode, of on average 0.051 eV.

Absolute photoabsorption cross sections and atmospheric photolysis
In this work we present the absolute photoabsorption cross sections of ethyl acetate measured for the first time. Since this compound is of environmental importance, due to being widely used as an industrial solvent, it is of interest to use these absolute cross sections in combination with solar actinic flux [33] measurements from the literature to estimate the photolysis rate of ethyl acetate in the atmosphere from an altitude close to the ground to the stratopause at 50 km. Details of the program used to perform the calculations are presented in a previous publication [34]. The quantum yield for dissociation following absorption is assumed to be unity. The reciprocal of the photolysis rate  [2] on ethyl acetate in aqueous phase that is of importance for acid rain chemistry. All these cases prove that not only is radical attack the main mechanism for the ester removal from the atmosphere, but also that the order of the reactivities of ethyl acetate with such species would be as follows: Cl ≈ OH NO 3 ≈ SO − 4 . UV photolysis is not expected to play a significant role in the tropospheric removal of ethyl acetate.

Conclusions
We present for the first time the complete electronic spectra of ethyl acetate, together with absolute photoabsorption cross sections from 4.5 to 10.7 eV. The structures observed in the spectrum were assigned to both valence and Rydberg transitions, based on ab initio calculations of vertical excitation energies and oscillator strengths of this molecule. Fine structure has been assigned to vibrational series involving predominantly excitations of ν 28 and ν 7 modes that are due to C−O and C=O stretches and OCC deformation combined with CO stretch and CH 3 rocking, respectively. In the He(I) photoelectron spectrum of ethyl acetate, vibrational excitations in the first ionic state were resolved and also assigned to ν 28 and ν 7 modes. The theoretical calculations presented here are in a good agreement with experimental data. Based on the photoabsorption cross section, photolysis lifetimes of ethyl acetate have been calculated for the Earth's troposphere and stratosphere and based on the literature values of rate constants for the reactions of ethyl acetate with hydroxyl, chlorine, nitrate and sulphate radicals, photolysis was excluded as a significant process of removal of this compound from the atmosphere.