Triple-Resonance Spectroscopy Reveals the Excitation Spectrum of

Letter pubs.acs.org/JPCL

Triple-Resonance Spectroscopy Reveals the Excitation Spectrum of Very Cold, Isomer-Specific Protonated Naphthalene Olha Krechkivska, Yu Liu, Kin Long Kelvin Lee, Klaas Nauta, Scott H. Kable, and Timothy W. Schmidt* School of Chemistry, The University of Sydney, New South Wales 2006, Australia S Supporting Information *

ABSTRACT: The excitation spectrum of very cold, isomerically pure protonated naphthalene is obtained by resonant dissociation spectroscopy. The cations are prepared by a pair of laser pulses which ionize 1-hydronaphthyl radicals at threshold, thereby creating only cations of a particular isomer in the vibrational ground state. Due to the small bandwidth of the first excitation laser, only the lowest rotational states are selected. The cold cation spectrum reveals a rich structure, some of which had previously been attributed to hot bands. However, the nature of the prepared cations is such that no hot bands appear, and the structure is assigned to a′ modes, and a′ combinations of a″ modes in the Cs point group. The 12 excited-state frequencies extracted agree well with those calculated previously at the RI-CC2 level. [Alata, I.; Dedonder, C.; Broquier, M.; Marceca, E.; Jouvet, C. J. Am. Chem. Soc. 2010, 132, 17483−17489.] SECTION: Spectroscopy, Photochemistry, and Excited States

F

induced dissociation. While not a general technique, this has been applied to obtain electronic spectra of several protonated PAHs.17−20 However, in the cited work, the cation is prepared in an electrical discharge, with no isomeric control. Furthermore, low temperatures are not guaranteed, which complicates the spectral assignment. In this contribution, we demonstrate the preparation of very cold, isomer-specific protonated naphthalene, by resonant 2photon at-threshold ionization of the corresponding radical. Subsequent resonant laser-induced dissociation of the cation completes the triple-resonance process to reveal the spectrum of 1-hydronaphthylium (Figure 1) in its ground vibrational state, and lowest rotational states. Resonant 2-color 2-photon ionization (R2C2PI) spectroscopy was performed on 1-hydronaphthyl radical in our previously described apparatus. Naphthalene was seeded into argon at 6 bar, and expanded into the source chamber through a pulsed discharge nozzle. The carrier gas was also seeded with water vapor (20 °C), which we have found to be an excellent source of hydrogen atoms, probably due to dissociative recombination with metastable argon atoms formed in the electrical discharge. The electrical discharge was struck for 100 μs, with 1.0−1.7 kV through a ballast resistor of 11 kΩ. The expansion containing the radical of interest was passed through a skimmer and between the grids of a time-of-flight mass spectrometer. Following the work of Zwier and co-workers, we measured the excitation spectrum of 1-hydronaphthyl radical.21,22 The

or over a quarter of a century, polycyclic aromatic hydrocarbons (PAHs) have been held responsible for infrared emissions from various circumstellar environments.1−4 They have also been suggested to carry certain unidentified absorption features in the spectra of starlight, due to intervening interstellar material.5,6 These so-called diffuse interstellar bands remain an astronomical mystery.7 The charge state of aromatic species in the interstellar medium remains an open question. However, modeling points to much of aromatic species being present in the cationic state.8,9 PAH cations can be open-shell species, such as the cations of commercially available PAHs (e.g., naphthalene, anthracene and phenanthrene), or closed-shell species, which are the cations of PAH radicals. One class of closed-shell aromatic cation may be described as protonated PAHs. Given the abundance of protons in interstellar space, these species are attractive as models for interstellar aromatic material.10,11 In order to compare a laboratory electronic spectrum to an astronomical measurement, the species of interest should be cold (few K), and isolated from other atoms, molecules and solvent.12 Gas-phase excitation spectra of aromatic cations have been measured by rare-gas tagging, wherein the loss of a weakly bound spectator atom such as Ar signals photon absorption by the chromophore.13,14 However, while such spectra are useful, they do not represent a rigorous comparison to interstellar molecules. In order to positively identify a DIB carrier, the spectrum of the isolated species is required. Cavity ringdown spectroscopy has been applied to some PAH cations, but without mass resolution, spectroscopic identification of species is less rigorous.15,16 Another pure gas-phase technique that offers the possibility of accurate band positions (if not intensities) is resonant laser© 2013 American Chemical Society

Received: September 16, 2013 Accepted: October 14, 2013 Published: October 14, 2013 3728

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Figure 1. Structure of protonated naphthalene (1-hydronaphthylium). The proton becomes indistinguishable from the other sp3-bonded hydrogen and the cation site is delocalized over the π-system.

resonant photons for the D1 ← D0 excitation were provided by a Nd:YAG-pumped dye laser and the excited radicals were ionized by the output of a frequency-doubled Nd:YAG-pumped dye laser tuned such that the sum of photon energies was equal to the ionization threshold of 6.57 eV.21 Threshold ionization ensures that only cations with zero vibrational quanta are formed. The resonant wavelength was set at the center of the origin band for the subsequent steps, ensuring that only the lowest few rotational levels were selected for ionization. Calibration was carried out with a wavemeter, which has previously been tested against an iodine absorption spectrum. The excitation resolution was about 0.08 cm−1. The strength of the two-laser dependent m/z 129 peak in the mass spectrum was monitored as a function of the wavelength of a third laser pulse, some 100 ns after the R2C2PI pulses. Where the ion signal was depleted, the cation was held to have absorbed, and subsequently dissociated. While we observed daughter ions (corresponding to H and H2 loss), the best signal was obtained by monitoring parent ion depletion. As will be shown, the ion-depletion was sufficient to record the excitation spectrum of very cold, isomer specific protonated naphthalene. A Jablonski diagam explaining the experimental scheme is illustrated in Figure 2. Quantum chemical calculations were carried out using the Gaussian09 program.23 The ground state geometry and frequencies were obtained at the B3LYP/6-311G(d) level,24−26 with the excited state equilibrium geometry and frequencies obtained using TD-B3LYP/6-311G(d).27 Calculated frequencies have been scaled by 0.97, in accord with our previous practice, for which we found to give superior agreement with experiment.28,29 The scale factor is empirical, and takes account of an incomplete treatment of correlation, and vibrational anharmonicity. A full list of calculated frequencies can be found in the Supporting Information. Zwier and co-workers reported the excitation spectrum of 1hydronaphthyl radical over a range of 2200 cm−1 with an origin band at 18949 cm−1.21,22 It is not our goal to repeat this work, but our spectrum is provided as evidence for resonant production of 1-hydronaphthylium cations. A comparison between our spectrum and that of Zwier and co-workers may be found in Figure 3. The spectra are essentially identical, with the present spectrum showing less saturation, as judged by peak heights relative to the origin. Our origin band is centered on 18950.6 cm−1, and has a width of just 2 cm−1 at half-maximum, indicating a rotational temperature of ∼5 K. The IE measured by Zwier and co-workers (6.57 eV) was confirmed, and this was used to prepare cations for measurement.

Figure 2. Experimental scheme. Jet-cooled, ground state radicals are resonantly ionized at threshold to yield an ensemble of isomer-selected cold cations. The cation signal is depleted by a resonant excitation pulse which induces a m/z change by dissociation.

Figure 3. The excitation spectrum of 1-hydronaphthyl radical. The top spectrum is adapted from refs 21 and 22, and the bottom spectrum is from the present work. The origin band at 18951 cm−1 was used to prepare cold cations.

The excitation spectrum obtained for 1-hydronaphthylium cation is shown in Figure 4, where it is compared to the lower resolution spectrum of Alata et al.17 Immediately clear is the much improved resolution. The bands in the present spectrum are about 2 cm−1 wide, on account of the jet-cooling of the radical, and the laser selection of low-J molecules. A simulation of the rotational contour observed for the origin band is provided in the Supporting Information. The experimental observation is satisfactorily simulated using the ground and excited-state rotational constants, and electronic transition moments calculated by (TD)-B3LYP, at a temperature of 5 K. A satisfactory fit was only obtained with a Lorentzian broadening of 1.1 cm−1, indicating a short-lived excited state (Δt ∼ 5 ps). It is worth noting that protonated benzene has such a short-lived excited state that no vibrational stucture is observed.30 This has implications for the fragmentation mechanism. The dissociation threshold for protonated naphthalene has been calculated to be 2.69 eV,31 which requires two photons at 503 nm (2.47 eV). If the first excited state lives for only 5 ps, 3729

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Figure 4. The excitation spectrum of 1-hydronaphthylium cation. The top spectrum is adapted from refs 17 and 18, and is aligned with the present spectrum to allow a comparison of the corresponding features.

the RI-CC2/cc-pVDZ calculations of Alata et al. predict an energy of about 160 cm−1. The observed band at 177 cm−1 may be thus assigned to 2ν51, there being no other possibility. The band at 263 cm−1 is assigned to 2ν50, and the band at 309 cm−1 is assigned to ν51 + ν49. Both assignments are similarly bracketed by the two methods of calculation. The next lowest a′ mode after ν34 is ν33, which is located at 465 cm−1. This assignment leaves the bands at 353 cm−1 and 454 cm−1 to be assigned to ν50 + ν49 and 2ν49 respectively. The band at 491 cm−1 is assigned to ν32, in agreement with Alata et al. The next mode, ν31, is assigned to the lower energy of the pair at 586 cm−1, those states being in a possible Fermi resonance. The higher frequency of the two bands has a relative frequency of 590 cm−1, which appears too low for 2ν48. However, the assignment ν49 + ν47 is sound, which leaves the feature at 544 cm−1 to be assigned to ν49 + ν48. Above 650 cm−1, the spectrum becomes increasingly difficult to assign. There are many plausible combinations and progressions, but suggested assignments are given in Table 1. From the assignments given, frequencies can be figured out for a″ modes, assuming (possibly erroneously) a certain degree of harmonicity. These are given in Table 2, with a comparison to the present calculations and those of Alata et al. The in-plane modes are also given, for ease of reference. What is strikingly

then the second photon absorbed in the 5 ns laser pulse is likely from a high-lying vibrational state on the S0 surface. The sequence-structure excitation spectrum from such states will mimic the cold spectrum, but will be broadened and shifted. However, the line-shape of the observed spectrum will remain essentially that of the one-photon excitation. Since the cations were prepared at threshold, via a resonant step involving the origin band of the radical, we can be sure that all of the structure obtained is due to the cold cations. Alata et al. ascribed some of their observed structure to hot-bands. However, we can be certain that our spectrum is hot-band free. The molecule is of Cs symmetry, and we commence assignment of bands to a′ vibrational states guided by quantum chemical calculations. Band positions and assignments are given in Table 1, along with the presently calculated frequencies and those of Alata et al.,17 for comparison. The lowest a′ mode is ν34, which is calculated to lie near 340 cm−1. The strong band at a relative frequency of 341 cm−1 is assigned to this mode, in agreement with Alata et al. Now, the three prominent bands to lower frequency than ν34 must be due to combinations of a″ modes. While they have been attributed to hot-bands by Alata et al., our method of cation preparation precludes this consideration. The lowest possible a′ vibrational state (barring the zero-point level) is 2ν51. The present TDB3LYP/6-311G(d) calculations put this above 200 cm−1, but 3730

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spectrum in a linear way for comparison (as opposed to multiphoton dissociation). By building on the present scheme, with an infrared pulse between the second and third laser pulses, ground state cations can be “shelved” into higher vibration states, and saved from the resonant destruction pulse. Such an experiment will reveal the linear infrared spectrum of the bare, cold cation, providing the sequence band generated does not overlap with the origin transition of the cation. A triple-resonance technique has been demonstrated to extract the excitation spectrum of isomer-selected cations. The spectroscopic selection ensures that only vibrational ground state cations are prepared, at low J. The cation spectrum is thus interpreted without having to consider hot-band assignments. In the present case of protonated naphthalene, or 1hydronaphthylium cation, the excitation spectrum was assigned, allowing the extraction of 12 vibrational frequencies. When compared to experiment, the RI-CC2 method implemented by Alata et al.17 was found to significantly out-perform TD-B3LYP for the out-of-plane modes. The demonstrated technique has several advantages: It is unambiguously isomer-specific, where the radical spectroscopy is known, and the cations themselves are very cold, making spectral assignments significantly easier. The technique is not general, since a mass-change is required at the final step, but for those cations where this does take place, this technique can be applied.

Table 1. Assigned Peaks in the Spectrum of 1Hydronaphthylium Cation (Figure 4) ν̅ 19869 20047 20133 20178 20210 20223 20323 20335 20360 20413 20456 20459 20540 20551 20562 20592 20596 20633 20702 20718 20796 20808 a

rel. ν̅ 0 177 263 309 341 353 454 465 491 544 586 590 670 682 693 722 727 764 832 849 926 938

assignment origin 2ν51 2ν50 ν51 + ν49 ν34 ν50 + ν49 2ν49 ν33 ν32 ν48 + ν49 ν31 ν47 + ν49 ν32 + 2ν51 2ν34 ν34 + ν50 + ν49 ν30 ν46 + ν48 ν29 ν32 + ν34 ν31 + 2ν50 ν31 + ν34 2ν33

calc.a

calc.b

0 210 248 333 337 365 455 459 492 557 596 619 702 675 702 726 791 758 829 870 933 918

0 166 275 299 336 340 431 464 487 527 585 562 653 672 676 732 729 768 823 833 920 928

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TD-B3LYP × 0.97 (this work). bRI-CC2 (ref 17).

a

mode

Γ

exp.

calc.a

calc.b

29 30 31 32 33 34 46 47 48 49 50 51

a′ a′ a′ a′ a′ a′ a″ a″ a″ a″ a″ a″

764 722 586 491 465 341 410 363 317 227 132 89

758 726 596 492 459 337 462 391 330 228 137 105

768 732 585 487 464 336 418 346 311 216 124 83

ASSOCIATED CONTENT

S Supporting Information *

Table 2. Normal Mode Frequencies Extracted from the Assignments in Table 1, Compared with the Present Calculations, and Those from Ref 17

A simulation of the observed rotational contour of the 1hydronaphthylium excitation spectrum; a complete list of frequencies for the ground (excited) state calculated by (TD)B3LYP/6-311G(d). This material is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +61 2 93 51 27 81. Fax: +61 2 93 51 33 29. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported under Australian Research Councils Discovery Projects funding scheme (DP120102559). KLKL acknowledges the receipt of an Australian Postgraduate Award.

TD-B3LYP × 0.97 (this work). bRI-CC2 (ref 17).

clear is that the RI-CC1 method of Alata et al. is superior for the out-of-plane modes in particular. The observed spectrum of 1-hydronaphthylium exhibits an origin band which peaks at 19869.3 cm−1, which corresponds to a wavelength in air of 5031.5 Å. The observed width of ∼2.5 cm−1 correponds to ∼0.6 Å, which is similar to many DIBs. The closest DIB is located at 5027 Å,32 and so if this species does cause interstellar light extinction, this is yet to be verified. The TD-B3LYP calculations predict an oscillator strength of just f = 0.0065. As shown by Duncan and co-workers, the infrared spectrum of protonated naphthalene shows many features that are consistent with the unidentified infrared bands (UIRs).11 However, their spectrum was obtained using spectator atoms and is not the true cation spectrum. While sufficient for their scientific purposes, it is also of interest to obtain the bare cation

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REFERENCES

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