UV and IR absorption spectra of C3 embedded in solid para-hydrogen

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Abstract

This paper presents the UV and IR absorption spectroscopy of small carbon molecules of C3 observed using a high-resolution Fourier-transform spectrometer. The C3 molecules were produced by irradiation of dimers or larger clusters of acetylene with an ArF laser (λ=193 nm). Sharp UV absorption features with multiple structures were observed in the Ã1ΠuX̃1Σg+ electronic transition of C3. The sharp UV absorption demonstrates the potential of solid para-hydrogen as a matrix for high-resolution spectroscopy of UV–vis electronic transitions.

Introduction

The small carbon molecules of C3 have received considerable attention from many spectroscopists since the first identification of the molecule by Herzberg [1]. The researchers’ interests cover not only the basic science but also its applications to chemical reactions in combustion flame and interstellar space [2], [3]. Both the ground X̃1Σg+ and the first excited Ã1Πu electronic states have been extensively studied in the gas phase [4], [5], [6], [7], [8], [9], [10], [11] as well as in rare gas matrices [12], [13], [14], [15], [16], [17], [18], [19], [20].

In this paper, we present and report on the high-resolution UV spectra of C3 embedded in solid para-hydrogen (p-H2). The popularity of solid p-H2 crystals as a host for matrix isolation spectroscopy has grown steadily in the last several years [21], [22], [23], [24]. The favorable properties of solid p-H2 as a matrix host include astonishingly sharp infrared absorption lines [24], [25] and feasibility with in situ photolysis [22]. For example, the high-resolution infrared spectroscopy of methane [26], [27] and methyl radicals [28] in solid p-H2 revealed fully resolved ro-vibrational spectra of the dopant molecules in single substitutional sites of a hexagonal-close-packed (hcp) crystal. So far, the application of high-resolution matrix isolation spectroscopy in solid p-H2 has been limited to the spectroscopy in the infrared (IR) region. The aim of the present paper is to demonstrate the applicability of the solid p-H2 matrix to the high-resolution spectroscopy of electronic transitions of dopant molecules as well.

In previous papers [29], [30], small carbon molecules Cn (n=3, 5, 7, and 9) produced by the laser ablation of graphite were successfully isolated in solid p-H2 matrices, and the IR absorption spectra of these molecules were identified. Herein, we present a high-resolution UV absorption spectrum of the Ã1ΠuX̃1Σg+ electronic transition of C3 in solid p-H2. The IR absorption spectra of C3 in solid p-H2 observed with higher spectral resolution than previous research [29], [30] are also discussed.

Section snippets

Experiment

In the present study, C3 molecules were produced by in situ UV photolysis of dimers or larger clusters of acetylene (C2H2) embedded in solid p-H2. The UV photolysis of C2H2 clusters was found to produce C3 molecules more efficiently than the laser ablation of graphite that we employed in previous research [29], [30]. We also performed UV photolysis of benzene (C6H6) as well as laser ablation of fullerene (C60) [31] to produce C3 molecules in solid p-H2. Irrespective of the precursor molecules,

Results of photolysis

In the present study, we produced C3 molecules by the UV photolysis of dimers or larger clusters of C2H2. The clusters of C2H2 in solid para-hydrogen turned out to yield C3 molecules quite efficiently. In addition to C3, many other molecules were produced after the photolysis. We first discuss products of the UV photolysis briefly in this section. The UV photolysis of acetylene dimers and trimers in Xe and Ar matrix has been studied extensively by Maier and Lautz [33].

The upper panel of Fig. 1

The band origin of the Ã1ΠuX̃1Σg+ transition

Fig. 3 shows the UV spectrum of C2H2/p-H2 system after irradiation with 193 nm photons. A strong absorption at 24,499 cm−1 (408 nm) is assigned to the band origin of the Ã1ΠuX̃1Σg+ transition, because the transition wavelength is close to that in solid Ar (410 nm) [12], in solid Ne (405 nm) [14], [20], and in the gas phase (405 nm) [7], [8], [9]. In Fig. 3, the band origin is designated as Ã(000)–X̃(000), where the numbers in parentheses (ν1ν2ν3) indicate three vibrational quantum numbers in

The ν3 fundamental

After the UV irradiation of the C2H2/p-H2 system, new absorption lines appeared in the C–C stretching region as shown in Fig. 5. The absorption feature is assigned to the ν3 fundamental of C3. A strong doublet at 2034.5 and 2035.9 cm−1 was observed with nearly equal intensity. The spectral width was less than 0.01 cm−1 FWHM. The separation of the doublet was 1.4 cm−1. The transition frequency and spectral pattern agree well with those in the previous studies in solid p-H2 [29], [30].

In addition

Discussion

In the ν3 infrared absorption spectra, a doublet with splitting of about 1.5 cm−1 was observed. Each component of the doublet showed different polarization dependence relative to the crystal axis as shown in Fig. 7. The lower frequency component intensified with the polarization of light parallel to the crystal axis, while the higher component intensified with perpendicular polarization. The polarization dependence, however, was not perfect compared with other molecules [26]. Under

Conclusion

The electronic absorption spectrum of C3 in solid p-H2 is reported for the first time. The band origin of the Ã1ΠuX̃1Σg+ transition of C3 was found to be 24,499 cm−1 (408 nm). The narrow linewidth (<1 cm−1) of the electronic transition allowed us to observe rich spectral structures. Several vibronic bands were also observed in the AX transition. Among them, the vibronic bands related to bending excitations were much broadened compared with the bands of stretching excitations. In the IR

Acknowledgements

This study was partially supported by Grant-in-aid for Scientific Research of the Ministry of Education, Science, Culture, and Sports of Japan.

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