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Astron. Astrophys. 319, 331-339 (1997)

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4. Results

4.1. Absorption spectrum of a perylene fragment

Looking for perylene cation fluorescence in our experiment, we found an unexpected signal towards the blue of the absorption wavelength. It could be explained either by multiple absorption of visible photons or by photoinduced fragmentation. To answer this question, we examined the laser energy dependence of this signal. The relationships between the fluorescence intensity and the intensities of both laser beams are plotted in Fig. 3. They show that four 248 nm-photons are absorbed, while only one dye-laser photon (slopes [FORMULA] 4 and [FORMULA] 1 respectively). This last result implies that the emitting species is different from the one which absorbs the dye laser. We conclude that a fragmentation is induced by absorption of the dye laser photon, leading to a fluorescent product which emits at wavelengths shorter than absorption wavelengths. This emission was then spectrally analyzed in an attempt to identify the fluorescent product. The emission spectrum obtained for an excitation at 721.7 nm is displayed in Fig. 4. The fluorescent emission of the C2 molecule in the well-known Swan bands series: [FORMULA] at wavelengths [FORMULA] 437, 470, 506, 550, 600, 660 nm can be easily recognized (see 4.2 below).

[FIGURE] Fig. 3. Relationships between the total fluorescence signal and a the excimer laser intensity, b the dye laser intensity at [FORMULA] = 6290.7 nm. The best fit lines have slopes of 3.96 and 0.89 respectively. Saturation can be seen in panel (a). Similar relationship was found for any resonant wavelength of the dye laser.
[FIGURE] Fig. 4. The emission spectrum of C2, after excitation of the X species at 721.7 nm. A Swan bands series is observed, with bands at 437, 470, 505, 550, 600 and 660 nm. In the inserted frame we show a spectrum taken at higher resolution (R [FORMULA] 3000) of the strongest band (v'=3 to v"=2) at 470 nm, exhibiting the P and R rotational branches. Also shown in this frame is a simulation (see text) of the P and R branches (Q-lines neglected) at a rotational temperature of 35 K.

We then attempted to check for the ionic character of the species responsible for the absorption. Application of an electric field as high as 50 V cm-1 was found to have no effect on the fluorescence signal. The obvious conclusion is that the parent molecules of the emitting C2 is a neutral species that cannot be identified at present stage, and we label X in the following.

The LIF excitation (absorption) spectrum of the X-species is shown in Figs. 5 & 6. It extends over the range 11,000-17,000 cm-1 (588-910 nm). The resolution of the tunable dye laser is of the order of 0.1 cm-1. This spectrum is characterized by several groups of bands separated by a few 100 cm-1 typically. Each group is divided into a set of bands which are about 1 cm-1 wide, and separated by a few 10 cm-1. The extension of a group is 50 to 80 cm-1. We found the pattern of these spectral features to be insensitive to the laser intensities spanned in our experiments (see Fig. 3). We understand this spectrum as a progression of vibronic bands whose origin lies at 12,134.7 cm-1 (824.0 nm). The wavenumbers of active vibrational modes can thus be found from the distances to the origin. The widths of the bands within a group are compatible with rotational broadening of a large species. The multiplicity of each vibrational group of bands is probably due to electronic fine structure. There are 2 additional features towards the red of the origin which we believe are hot bands. We have tested this interpretation by trying to change the efficiency of collisional cooling in the jet. First, rising the backing pressure of the carrier gas increases the cooling of sample molecules. Second, for what regards the carrier gas, argon is a better coolant than helium as already mentioned (Sect. 2). Consistently with the hot band hypothesis, we observed that increasing the backing pressure and using argon in the jet resulted in weaker bands at 11,488 and 11,823 cm-1.
The small widths of individual vibronic bands (1 cm-1 and less) correspond to a rotational temperature of a few K (Cossart-Magos & Leach 1990), as well as the weak intensity of the hot bands demonstrates that the X-species vibrational modes are efficiently cooled by collisions with the carrier gas between the excimer and dye laser excitations.
The strong [FORMULA] -band starting at 15,835 cm-1 may be the origin of a new electronic transition.

[FIGURE] Fig. 5. Excitation (or absorption, see Sect. 2) spectrum of the X-species. It was recorded by measuring the whole fluorescence of C2 in the strong 470 nm Swan band, as a function of the dye laser wavelength. The resolution of the dye laser is 0.1 cm-1. 7 different dye solutions, and frequency shift by stimulated Raman scattering in H2 have been necessary to cover this wide region (588-910 nm). We assign the main origin at 12,134.7 cm-1. Another one is probably seen at 15,835 cm-1. Two different groups of vibronic bands are labeled by [FORMULA] and [FORMULA] (see Sect. 4.1).
[FIGURE] Fig. 6. Blown-up view of the first 2 vibronic bands representative of [FORMULA] and [FORMULA] -type respectively. Frequency spacings of the characteristic features of the pattern are labeled as [FORMULA], their values are listed in Table 2.

The third column of Table 1 gives the wavenumbers, taken from the origin, of the vibrational frequencies extracted from the excitation spectrum. Also given are the vibrational modes for neutral perylene (first column) and perylene cation (second column) that were found to match the X frequencies within about 50 cm-1. It is remarkable that 9 modes among 15 of the X-spectrum are also present in C20 H12 and/or C20 H [FORMULA]. At frequencies larger than 2000 cm-1 the lack of measurements in the literature did not allow us any more comparison. In particular, the low-frequency butterfly mode of neutral perylene at 94 cm-1 is also present in the X-spectrum. From this we infer that the X-species must have an overall structure close to that of perylene, i.e., X is probably a large perylene fragment, possibly a partially dehydrogenated perylene. This is coherent with the result of photofragmentation experiments (Jochims et al., 1994, P. Boissel, private communication): perylene preferentially ejects a H2 fragment after a 18 eV irradiation; this result observed for cations should also be valuable for neutral species.


[TABLE]

Table 1. Frequencies (in cm-1) of the vibrational modes of the unknown X-radical taken from the origin at 12,134 cm-1 together with those of neutral perylene and its cation, when these latter match the X frequencies, within 50 cm-1. References for the measurements are: (a) states of the neutral ([FORMULA] and [FORMULA]) by Fourman et al. (1985), Wittmeyer and Topp (1993) (jet), infrared active modes by Szczepanski et al. (1993) (Ar matrix), (b) ground electronic state of the cation ([FORMULA]) by Joblin et al. (1995) (Ne matrix), Negri and Zgierski (1994) (theory), infrared active modes by Szczepanski et al. (1993) and (c) present work (jet).


Fig. 6 shows an enlarged view of the excitation spectrum near the origin. Two different types of vibronic bands are distinguishable: (i) the origin itself, composed of 8 strong lines and (ii) the first vibronic band whose structure is much simpler with only 6 lines, and 2 major ones. All vibronic bands can be cast into these two different groups, which we label respectively [FORMULA] -bands (bands alike the origin) and [FORMULA] -bands (bands alike the first vibronic transition) (see Fig. 5). Fine structure intervals in each family vary slightly from one vibrational state to another as may occur in spectra of complex molecules where the density of states is high. We report in Table 2 the measured intervals, [FORMULA], taken from the origin of each vibronic band (see Fig. 6). As can be seen, all the lines of the [FORMULA] -bands are present in [FORMULA] -bands with the exception of [FORMULA]. On the other hand, the [FORMULA] and [FORMULA] -lines of [FORMULA] -bands are absent from the [FORMULA] -ones. Fine structure intervals are very similar (with a mere 10% deviation) in each band-class over 5,000 cm-1.


[TABLE]

Table 2. We give here electronic fine structure line intervals taken from the origin of each vibronic band. See Figs. 5 and 6 for definition of [FORMULA] -bands and [FORMULA] -bands.


We determined the temperature of the X-species in the jet with the code of Birss and Ramsay (1984), which computes the rotational spectrum of an asymmetric rotor. We used the exact values of perylene rotational constants, assuming a similar geometry for the X radical. The profiles are more symmetrical when the transition moment is taken in the plane formed by a and b axis. The measured widths in the spectrum (Fig. 6) are of the order 0.8-1 cm-1, corresponding to a rotational temperature in the jet of 3-4 K. The species are thus very efficiently cooled.

4.2. [FORMULA] emission

Fig. 4 presents the emission spectrum observed when exciting the X-species at 721.7 nm. The main spectrum shows some features at positions close to 437, 470, 505, 550, 600, and 660 nm. These emission bands are independent of the excitation wavelength, from 824 up to 585 nm. We identified these bands to the Swan band series of the C2 molecule (Danylewych and Nicholls, 1974). Indeed, positions and intensity ratios are coherent with the initial vibrational level being [FORMULA]. A very selective excitation of [FORMULA] occurs, that leaves the fragment in the [FORMULA] state. Furthermore, our lifetime measurements from a digital oscilloscope (125 [FORMULA] 10 ns) are also consistent with litterature data (Naulin et al. 1988).
The frame inserted in Fig. 4 shows the strongest 470 nm band at higher resolution ([FORMULA] = 0.15 nm). A substructure with two bands is visible, which we interpret as rotational P and R branches. The Q branch is too faint to be seen. We model this rotational contour, using molecular constants of Dhumwad et al. (1981), and find a rotational temperature of [FORMULA] 35 K to fit the observed width and intensity ratio between P and R branches. The small [FORMULA] fragment is thus ejected with a relatively small rotational energy. This is consistent with a statistical unimolecular decay process. However, the origin of the selective vibrational excitation is unknown.

4.3. Fragmentation process

In this section, we outline the energetics and characteristic time constants involved in the formation of the X-species.

As discussed in Sect. 4.1, the parent molecule, perylene, sequentially absorbs 4 excimer photons at 248 nm in the sequence of processes leading to some species which fragments by absorption of visible light. The first absorption corresponds to a [FORMULA] -transition (or [FORMULA] -band, Birks 1970). Assuming an average absorption cross-section of 1.10-16 cm-2 (Joblin et al. 1992), it can be estimated that perylene absorbs a 5 eV-photon every 40 ps in the excimer beam. After the first 5 eV-excitation, perylene can relax only via fluorescent and/or vibrational cascades which both have longer timescales: a few nanoseconds at least and a few tenth of seconds, respectively (Léger et al. 1989b and references in Jochims et al. 1994). Indeed, no photofragmentation takes place in a short timescale in PAHs for internal energies below [FORMULA] 7 eV (Ling and Lifshitz 1995, Jochims et al. 1994 and Gotkis et al. 1993). The second 5 eV-photon thus excites the molecule at 10 eV above ground level. At such excitation energy, ionization and/or fragmentation processes can proceed efficiently. They will lead to either a radical R [FORMULA] or an ion I [FORMULA] (see Fig. 7) bearing at most an excitation energy of a few eV. This species will in turn, just as earlier, absorb sequentially two 5 eV-photons and give rise to a new species. If the intermediate is an ion I [FORMULA], it will necessarily fragment since the loss of a second electron costs too much energy: the resulting heavy fragment would of course keep the charge. Should the intermediate be a neutral R [FORMULA], it may ionize or fragment into X [FORMULA]. Since the experiment shows that our X-species is neutral the last process applies to our case. It is then efficiently cooled by the carrier gas (see Sect. 4.1) and relaxes to its ground state, X. Finally, upon absorption of a visible photon (over the range 580-820 nm), the X-radical suffers a new fragmentation and the C2 -molecule is ejected, very selectively excited in the level [FORMULA]. This sequence of fragmentation processes is described in Fig. 7.

[FIGURE] Fig. 7. Fragmentation sequence occuring in our experiment. Perylene sequentially absorbs 4 photons at 248 nm or equivalently two 10 eV-photons (see Sect. 4.2). After the first 10 eV-absorption an excited radical R [FORMULA] or ion I [FORMULA] is formed. It then absorbs 10 eV to yield another excited radical, X [FORMULA]. This latter is cooled by collisions with the carrier gas and yields the X-species. Finally X absorbs a visible photon and dissociates, producing C2 very selectively excited in the [FORMULA] state. The fluorescence we observe emanates from the [FORMULA] 1 to 7)-transition (known as Swan bands).
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© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998
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