Vacuum-Ultraviolet Absorption Spectra of Icy C2H4 at 13–60 K

The thermal variation of absorption spectra of icy ethene in wavelength range 105–220 nm was measured from 13 to 100 K using a synchrotron as light source. Sublimation of icy ethene began above 62 K, resulting in decreasing absorption. The absorption of icy ethene increased at wavelengths less than about 150 nm with increasing temperature from 13 to 60 K, but decreased beyond above 150 nm. According to detailed examination, the absorption spectra of icy ethene intersected at isosbestic point 147.0 nm from 13 to 17 K, whereas those varied absorption profiles crossed at another point, 150.6 nm, from 23 to 60 K. These results indicate that ethene ices might exhibit three structures within temperature range 13–60 K. This work enhances our understanding of the spectra of icy ethene at low temperatures and our knowledge of its astrochemistry and astrophysics in cold astro-environments.


INTRODUCTION
Absorption spectra of condensed and gaseous molecules typically exhibit profiles that depend on the response to the presence or absence of interaction of the molecule with other adjacent molecules. The gaseous molecule behaves as isolated, with no perturbation from other molecules; its spectral profile hence reveals the intrinsic properties of a single molecule. In contrast, the molecules in condensed phases are inevitably involved in intermolecular interactions with adjacent molecules; their spectra hence invariably involve the collective effects of the molecules in the condensed state (Lu et al., 2005;Chou et al., 2018;Lo et al., 2018).
The absorption spectra of molecules might exhibit a temperature effect associated with the couplings of vibrational and rotational transitions; the thermal variations of spectra of molecules are thus interesting for both research and applications. The thermal spectral responses of condensed and gaseous molecules might thus differ with or without the interaction of adjacent molecules. The thermal variation of absorption for a molecule in the gaseous phase (Lu et al., 2010;Cheng et al., 2011) is generally greater than that in a condensed state; for the thermal effect on spectra, attention has been devoted much to the gaseous molecule and little to condensed molecules. For this reason, an investigation of the thermal absorption for molecules in condensed phases is lacking, and needs to be established.
Ethene, C 2 H 4 , is the simplest stable hydrocarbon molecule with a double bond. It has been detected in Jupiter, Saturn, Titan, Neptune, Pluto, comets and star IRC +10216 (Encrenaz et al., 1975;Betz, 1981;Hanel et al., 1981;Goldhaber et al., 1987;Kostiuk et al., 1993;Schulz et al., 1999;Fonfría et al., 2017); observations of icy ethene have been reported elsewhere in cold space. Vacuumultraviolet (VUV) radiation is an important driving force for the evolution in the universe of astromolecules. To understand the VUV photochemistry of ethene in cold astro-environments, it is first necessary to study the VUV absorption of icy ethene in the laboratory. The melting point of pure ethene is 104 K; condensed solid ethene can thus exist below 100 K in cold space. To investigate this system, we thus recorded the VUV absorption spectra of icy ethene in wavelength range 105-220 nm from 13 to 100 K; for explanation the results, we suggest that ethene ices might contain three icy structures at low temperature. Besides the UV/VUV data, also IR absorption spectra are presented to strengthen the discussion about the changes in the icy structure, by comparison with available literature (Hudson et al., 2014). The results enhance our understanding of the spectra of icy ethene in cold astroenvironments.

MATERIALS AND METHODS
We recorded the absorption spectra of pure solid C 2 H 4 in the ultraviolet (UV) and vacuum-ultraviolet region to the limit of transmission of optical components, about 105 nm. The apparatus is described elsewhere (Lu et al., 2005;Chou et al., 2014). In short, the UV/VUV absorption spectra were recorded at an end station attached to synchrotron beamline 03 (BL03), which was coupled to the 1.5 GeV storage ring at Taiwan Light Source (TLS) in National Synchrotron Radiation Research Center (NSRRC). The BL03 is appended a cylindrical-grating monochromator (CGM, 6 m) equipped with four gratings; in this work, we used a grating (450 L/mm) to provide light suitable for wavelength range 100-250 nm. The intensity of the UV/VUV light was monitored with a gold mesh transmitting about 90% and recorded with an electrometer (Keithley 6512). The radiation was incident on samples in a beam of nearly rectangular cross section, ∼8 × 5 mm 2 , which delivers a mean flux 2 × 10 11 photons s −1 after passage through a LiF window as harmonic filter. The storage ring of the synchrotron was operated to maintain a constant beam current of electrons by means of injection of a few electrons into the ring every minute; the electron current hence maintained 99.5 percent stability.
The light dispersed from BL03 irradiated the icy sample on a LiF window cooled with a cryostat. The UV/VUV light was incident on the icy sample in a perpendicular beam. The transmitted light impinged on a glass window coated with sodium salicylate; the fluorescence was detected with a photomultiplier tube (Hamamatsu R943) in a photon-counting mode (Lu et al., 2020). The absorption spectra were recorded at spectral resolution 0.2 nm. C 2 H 4 was deposited onto the LiF window, attached to a rotary cryostat (APD HC-DE204S), which could be cooled to 13 K as minimum temperature. A turbo-molecular pump backed with a scroll pump evacuated the chamber of the end station to a pressure less than 8 μPa. For UV/VUV absorption measurement, the gaseous C 2 H 4 was deposited onto the LiF substrate at 13 K. The quantity of deposition was regulated at the pressure 25 μPa for a period of 60 s; the sample film was estimated in thickness less than 0.3 μm.
For IR absorption measurement, the spectra were recorded with the end-station attached to the beamline BL21A2 at TLS Lo et al., 2019). A closed-cycle cryogenic refrigerator (Janis RDK-415) employed to cool the KBr substrate; in which, the cryo-chamber was evacuated with a turbomolecular pump under the pressure less than 4 μPa. The rate of deposition was regulated at 2.2 μmol s −1 for a period of 160 s; the thickness of the film was estimated about 2 μm. IR spectra were recorded at various temperatures with an interferometric spectrometer (Bomem DA8) equipped with a HgCdTe detector to cover the spectral range 500-4,000 cm −1 at resolution 0.5 cm −1 ; typically, the IR spectra were accumulated with 400 scans. We recorded spectra at the minimum temperature 8 K and then after increasing the temperature to selected values; which were controlled by a temperature controller (LakeShore 340) with a rising rate 1 K/min. Gaseous C 2 H 4 was purchased from Matheson Gases with a nominal purity 99.99%; by passing through a cold trap maintained at 180 K, the gaseous C 2 H 4 were deposited onto the LiF and the KBr substrates at 13 and 8 K, respectively. Figure 1A displays the absorption spectrum, in terms of cross section (Mb 10 -18 cm 2 /molecule), of gaseous C 2 H 4 in wavelength range 105-185 nm at 298 K. Gaseous C 2 H 4 possesses strong absorption in the VUV region (Lu et al., 2004); the absorption curve exhibits characteristic features associated with Rydberg transitions in series in wavelength range 105-185 nm. Among these absorption lines, the features in wavelength region 150-180 nm are due to the 3s Rydberg transition with maximum cross section 58 Mb at 170 nm, whereas those occurring in region 130-140 nm correspond to 3d and 4d Rydberg transitions. Absorption lines from 130 to 118 nm are attributed to overlapping transitions to 4d, 5d, 5s, and 6s Rydberg states (Zelikoff and Watanabe, 1953;Wilkinson and Mulliken, 1955;Person and Nicole, 1968;McDiarmid, 1980;Xia et al., 1991;Holland et al., 1997). The absorption cross sections of   Figure 1B depicts the absorption spectrum of solid C 2 H 4 in wavelength range 105-240 nm near 10 K. From a comparison of the two spectra in Figure 1, the absorption profiles for gaseous and solid C 2 H 4 reveal great differences. In contrast to the gaseous absorption features, the absorption of solid C 2 H 4 near 10 K appears as a continuous curve without distinct lines. The absorption threshold wavelength of solid C 2 H 4 occurs about 220 nm as shown in the insert of Figure 1; at a wavelength shorter than that threshold in region 105-200 nm, the absorption of solid C 2 H 4 comprises two broad bands, with maxima about 172 nm (with a shoulder about 150 nm) and 125 nm. A weak vibronic progression of the solid C 2 H 4 appears in the wavelength region 190-165 nm; the average separation of this progression is about 670 ± 70 cm −1 .

RESULTS AND DISCUSSION
The spectral profile of the absorption of icy C 2 H 4 might exhibit a thermal effect, but no thermal variation of its spectra has been reported, despite the need of its establishment. Considering that the melting point of C 2 H 4 is 104 K, we thus measured the thermal variation of VUV absorption spectra of icy C 2 H 4 from 13 to 100 K; as a result of sublimation of icy C 2 H 4 , the intensity of the absorption began to decrease, by about half at 65 K, with none remaining at 80 K. As little sublimation of icy C 2 H 4 occurred in range 13-60 K (see Supplementary Figure S1 in Supplementary Material), we thus could examine the thermal variation of its VUV absorption in this temperature range. Figure 2 displays the VUV absorption spectra of icy C 2 H 4 recorded at temperatures 13,15,17,19,23,25,35,45,55, and 60 K, in wavelength range 105-245 nm. Notably, the VUV absorption spectra of icy C 2 H 4 was not constant but exhibited a significant thermal effect in temperature range 13-60 K, as shown in Figure 2. Unexpectedly, the thermal variation of the absorptions of icy C 2 H 4 revealed two distinct trends in wavelength range 105-200 nm: the intensity of absorption for a wavelength less than about 150 nm increased with temperature increasing from 13 to 60 K, whereas absorption beyond about 150 nm decreased as the temperature was analogously raised.
Upon close examination, the thermal variation of absorption profiles for icy C 2 H 4 could be divided into two temperature ranges 13-17 K and 23-60 K. Figure 3 presents the absorption spectra of icy C 2 H 4 at temperatures 13, 15 and 17 K. Remarkably, these three absorption curves intersect at wavelength 147.0 nm, which is thus an isosbestic point of thermal variation of absorption spectra for icy C 2 H 4 in temperature range 13-17 K. This result hints that C 2 H 4 ices might possess two distinct FIGURE 2 | Absorption spectra of icy C 2 H 4 recorded at temperatures 13,15,17,19,23,25,35,45,55, and 60 K in wavelength range 105-245 nm.  structures in temperature range 13-17 K, which could exchange under an equilibrium condition in this temperature range. Figure 4 presents the absorption spectra of icy C 2 H 4 at temperatures 23, 25, 35, 45, 55, and 60 K. Extraordinarily, these absorption spectra intersect at 150.6 nm, which becomes another isosbestic point of thermal variation of absorption spectra of icy C 2 H 4 in temperature range 23-60 K. This result indicates that C 2 H 4 ices might also have two distinct structures in temperature range 23-60 K; in this temperature range, the structures of these two ices could also interconvert in an equilibrium condition. The thermal variations of absorption profiles of Figures 3, 4 thus display separate isosbestic points 147.0 and 150.6 nm; these results imply that ices of the two kinds involved in each equilibrium condition in these two temperature ranges might not be the same.
To rationalize these observations of the thermal variation of absorption spectra of icy C 2 H 4 in temperature range 13-60 K as shown in Figures 2-4, we propose that there are structures of icy C 2 H 4 of three kinds, so C 2 H 4 (ice I ), C 2 H 4 (ice II ), and C 2 H 4 (ice III ) could form in temperature range 13-60 K. The VUV absorptions of C 2 H 4 (ice I ), C 2 H 4 (ice II ), and C 2 H 4 (ice III ) might have their own features and be dissimilar. After deposition, the total number of icy C 2 H 4 molecules (N ice ) should be the same in the entire range 13-60 K; the total number of these three structures of ices --N iceI , N iceII and N iceIII --is constant and is equal to N ice at all temperatures of 13-60 K in this system as Eq. 1. (1) As displayed in Figures 3, 4, all curves intersect at only one point in each case in these absorption spectra for varied temperature; these phenomena indicate that only two species exist in a condition of rapid equilibrium upon thermal changes in each of Figures 3, 4. In range 13-17 K of Figure 3, only C 2 H 4 (ice I ) and C 2 H 4 (ice II ) might occur in equilibrium as in Eq. 2.
In this range 13-17 K, as no C 2 H 4 (ice III ) ice could form, N iceIII is zero; the mass Eq. 1 thus becomes Eq. 3.
In contrast, only C 2 H 4 (ice II ) and C 2 H 4 (ice III ) might exist in Figure 4 as in Eq. 4 in temperature range 23-60 K; mass Eq. 1 then becomes Eq. 5.
Consequently, C 2 H 4 (ice II ) might persist in the entire temperature range 13-60 K, but C 2 H 4 (ice I ) could exist simply below 23 K whereas C 2 H 4 (ice III ) might barely survive above 17 K. In the temperature range 13-17 K, C 2 H 4 (ice I ) and C 2 H 4 (ice II ) contributed the varied absorption spectra with conditions of Eqs. 2, 3; as a result, these spectral curves intersect at 147.0 nm as shown in Figure 3. C 2 H 4 (ice II ) and C 2 H 4 (ice III ) exhibited their thermal spectra merely in conditions according to Eqs. 4, 5 in temperature range 23-60 K; these absorption curves intersect at 150.6 nm as shown in Figure 4.
Based on this scheme, ices C 2 H 4 (ice I ), C 2 H 4 (ice II ) and C 2 H 4 (ice III ) coexist in temperature range 17-23 K; accordingly, the absorption curve of icy C 2 H 4 measured at 19 K as shown in Figure 2 belonged to neither the group as displayed in Figure 3 nor that in Figure 4.
Icy C 2 H 4 (ice I ), C 2 H 4 (ice II ) and C 2 H 4 (ice III ) respond to their own characteristic spectral features; which might also be occurred in the IR spectroscopy. Figure 5 displays the IR absorption spectra of icy C 2 H 4 recorded at temperatures (a) 8 K, (b) 10 K, (c) 12 K, (d) 14 K, (e) 18 K, (f) 20 K, (g) 30 K, (h) 40 K, (i) 50 K, (j) 55 K, (k) 60 K, (l) 65 K and (m) 70 K, respectively. Compared the IR absorption intensity at 65 K ( Figure 5L) to at 60 K ( Figure 5K), its value decreased; the result hints that the icy C 2 H 4 could sublime above 60 K and explains that no IR signal was detected due to total evaporation away at 70 K as shown in Figure 5M. From Figure 5A, the positions of vibrational lines with greatest absorbance for icy C 2 H 4 are at 941.6/949.6 (] 7 ), 1,435.0/1,436.5/1,440.3 (] 12 ), 2,972.8 (] 11 ) and 3,088.1 (] 9 ) cm −1 at 8 K. Increasing the temperature, the IR lines of C 2 H 4 shift to blue with a narrowing effect from 8 to 60 K; for examples, the positions of the maximum intensity for the ] 9 and ] 11 modes appear to be 3,088.1 cm −1 (width 2.76 cm −1 ) and 2,972.8 cm −1 (width 1.90 cm −1 ), respectively, at 8 K (see Supplementary  Figures S2, S3 in the Supplementary Material); those values shift to 3,088.4 cm −1 (width 1.95 cm −1 ) and 2,973.2 cm −1 (width 0.99 cm −1 ) at 60 K. For the ] 12 mode at 8 K as shown in the insert of Figure 5, a broad triple were observed at 1,435.0, 1,436.5 and 1,440.3 cm −1 ; those gradually congested to a doublet at 1,436.4 and 1,440.6 cm −1 above 55 K and reveals the sharpest at 60 K.
Investigations of the structural changes for the icy C 2 H 4 have been reported at different temperatures previously (Hudson et al., 2014;Satorre et al., 2017). For this purpose, Hudson et al. measured the IR spectra of the icy C 2 H 4 at 12, 20, 30, 35, 40, 50, and 60 K (Hudson et al., 2014); our IR spectra recorded at various low temperatures are in satisfactory agreements with theirs. Hudson et al. concluded that the icy C 2 H 4 could form in amorphous, metastable, and crystalline structures; which are dominate at 12-20 K, 20-40 K and 50-60 K, respectively. Figure 5 supports their conclusion that there are three structures for icy C 2 H 4 at low temperature. Accordingly, we suggest that the icy C 2 H 4 possesses the amorphous structure at 8-20 K; increasing the temperature, the icy structure might involve the metastable phase at 15-50 K and change in most crystalline form above 55 K. Based on the thermal variation of VUV absorptions of icy C 2 H 4 , we propose that structural ices C 2 H 4 (ice I ), C 2 H 4 (ice II ) and C 2 H 4 (ice III ) of these three kinds existed in temperature range 13-60 K as discussed above. Our results indicate that ices C 2 H 4 (ice I ), C 2 H 4 (ice II ) and C 2 H 4 (ice III ) might exhibit their presence in temperature ranges 13-23 K, 13-60 K and 17-60 K, respectively. Referred to the Hudson et al's work, the structures of C 2 H 4 (ice I ), C 2 H 4 (ice II ) and C 2 H 4 (ice III ) might be in amorphous, metastable, and crystalline structures. The relative contents of these ices thus depend on the specific temperature. The chemical and physical properties of C 2 H 4 (ice I ), C 2 H 4 (ice II ) and C 2 H 4 (ice III ) might respond to their own characteristic features; these properties might influence the astrophysical and astrochemical reactions of icy C 2 H 4 in cold space. In future, it is then interesting to investigate the chemical and physical properties of C 2 H 4 (ice I ), C 2 H 4 (ice II ) and C 2 H 4 (ice III ) individually.

SUMMARY
We measured the absorption spectra of icy ethene in wavelength region 105-245 nm in temperature range 13-100 K using light from a synchrotron. We observed the curves for the thermal variation of spectra of icy ethene to intersect at 147.0 and 150.6 nm in temperature ranges 13-17 K and 23-60 K, respectively. Our results indicate that ices of three kinds C 2 H 4 (ice I ), C 2 H 4 (ice II ) and C 2 H 4 (ice III ), exist in temperature range 13-60 K; these ices might have their own absorption curves and are distinct. This work enhances our perception of icy ethene at low temperatures; we suggest to investigate this issue for its impact on astrochemistry and astrophysics in cold astroenvironments.

DATA AVAILABILITY STATEMENT
The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author.