Accurate Modeling of Infrared Multiple Photon Dissociation Spectra

Article pubs.acs.org/JPCA

Accurate Modeling of Infrared Multiple Photon Dissociation Spectra: The Dynamical Role of Anharmonicities Pascal Parneix,*,† Marie Basire,†,¶ and Florent Calvo‡ †

Institut des Sciences Moléculaires d’Orsay, UMR CNRS 8214, Université Paris Sud 11, Bât. 210, F91405 Orsay Cedex, France Institut Lumière Matière, UMR CNRS 5306, Université Lyon I, 43 Bd du 11 Novembre 1918, F69622 Villeurbanne Cedex, France

‡

ABSTRACT: The dynamical response of a molecular system to a macropulse typically produced by a free-electron laser is theoretically modeled over experimentally long times, within a realistic kinetic Monte Carlo framework that incorporates absorption, stimulated emission, spontaneous emission, and dissociation events. The simulation relies on an anharmonic potential energy surface obtained from quantum chemistry calculations. Application to cationic naphthalene yields a better agreement with measurements than the anharmonic linear absorption spectrum, thus emphasizing the importance of specific dynamical effects on the spectral properties.

1. INTRODUCTION Infrared multiple photon dissociation (IRMPD) has become a widely used experimental technique in many fields of chemistry and molecular physics.1−4 The advent of intense and tunable laser sources such as free-electron lasers (FELs), together with the selectivity and versatility offered by mass spectrometric methods and electromagnetic storage devices, can now be exploited to determine vibrational spectra of well-defined isolated molecules, making IRMPD invaluable as a structural probe in the gas phase. Besides experimental progress, the success of IRMPD spectroscopy is also due to quantum chemical advances, assignment being usually achieved based on electronic structure calculations of the absorption spectrum for candidate structures. Although standard, this procedure is known to have several deficiencies, most notably the need to correct the harmonic spectra a posteriori by one or several scaling factors.5 Scaling phenomenologically accounts for anharmonicities, vibrational delocalization, and basis set incompleteness and can be partly circumvented by performing anharmonic calculations, either perturbatively or by following the time-resolved molecular dynamics. One problematic issue is that IRMPD spectra are compared to calculated linear absorption spectra, despite originating from very complex mechanisms involving sequential photon absorption, stimulated and spontaneous emission, energy redistribution, as well as fragmentation, all with their own distinct time scales. Being an action technique in which the measured signal is a depletion ratio, it remains very unclear to which extent the two spectra should generally bear similarities. In some cases,6−8 clear differences with the IRMPD spectrum have been reported in the line shifts, the broadenings, as well as band intensities even when anharmonicities were accounted for in the linear absorption spectrum. These residual discrepancies emphasize the likely importance of kinetic processes that are specific to the IRMPD technique. © 2013 American Chemical Society

The dynamical role of anharmonicities on IRMPD spectra has been investigated on a few occasions;9,10 however, these studies used empirical information in order to reproduce experimental data. In this article, we present a fully consistent theoretical modeling of IRMPD spectra that does not rely on explicit experimental or phenomenological inputs. This model includes the most important physical mechanisms involved in IRMPD spectroscopy and treats them with accurate ingredients obtained from quantum chemistry data. As a test case, we have applied the model to cationic naphthalene (C10H+8 ), whose IR spectrum has been characterized under IRMPD,11 matrix,12 and messenger-tagging13 conditions. This molecule is archetypal of larger polycyclic aromatic hydrocarbons, which are of notable astrophysical interest since the so-called aromatic infrared bands were first observed in the interstellar medium.14 The model is described in section 2, and applied to cationic naphthalene in section 3 before some concluding remarks are finally given in section 4.

2. MODEL Our modeling is event-driven and consists of simulating the time evolution of the molecule under a FEL macropulse and taking into account photon absorption, stimulated emission, spontaneous emission, and dissociation all within a kinetic Monte Carlo (kMC) statistical framework. In this model, depicted in Figure 1, the laser profile consists of successive macropulses with a duration tpulse of 10 μs, separated by a much longer period of about 0.1 s. This time is very long with respect to molecular, radiative, and even dissociative processes, and as experimentally justified,9 we can safely limit our description to a single macropulse and the subsequent relaxation kinetics. We Received: March 11, 2013 Revised: April 11, 2013 Published: April 12, 2013 3954

dx.doi.org/10.1021/jp402459f | J. Phys. Chem. A 2013, 117, 3954−3959

The Journal of Physical Chemistry A

Article

about 1 ns without being exposed to laser light until the next micropulse occurs. As a consequence, the microscopic rates should be determined in the microcanonical ensemble. At the current time t, all possible events {α} of photon absorption, stimulated emission, spontaneous IR emission, and molecular dissociation are enumerated with their individual rates kα at the current internal energy E. This requires photon energies ν to be discretized. As in standard kMC, one event α is then randomly drawn based on its normalized probability pα = kα/ktot, with ktot = ∑γ kγ = 1/τ(E). The selected event is performed, with the internal state n being updated accordingly in the case of emission or absorption. The previous state of the system is associated with a residence time of Δt = −τ × (ln ρ), with 0 < ρ ≤ 1 as a random number. Unless dissociation has happened, the time counter is updated as t → t + Δt, and the simulation proceeds. As soon as time exceeds tpulse, only spontaneous IR emission occurs in competition with dissociation. The absorption and emission rates ka, ks, and ke all depend on internal energy E and on the photon energy hν. Absorption (a) and stimulated (s) emission can be treated on a common footing, with rates ka/s(ν,E) = σa/s(ν,E) × ϕ(ν), where σa/s is the corresponding cross section. The dependencies of these cross sections on both E and ν are evaluated by multicanonical Monte Carlo.18 In these simulations, the current state n with energy E contributes to the two-dimensional histograms σa/s(ν,E) by the quantity σ(nk → nk ± 1), summing over each single-photon transition of mode k from nk to nk ± 1 (+ for absorption, − for emission) with associated energy ΔEa/s. The harmonic approximation relates the oscillator strength σ(nk → nk ± 1) to the Einstein coefficient σ(0 → 1), and we bin these contributions according to the photon energy hν as

Figure 1. Schematic representation of the FEL photon flux profile and list of possible events occurring to the system during and after the macropulse, with their associated rate constants.

further neglect the micropulse structure of the macropulse in such a way that the normalized photon flux ϕ at frequency ν is modeled as ⎡ (ν − ν )2 ⎤ Φ0 L ⎥ exp⎢ − 2π ΔνL 2ΔνL2 ⎦ ⎣

ϕ(ν) =

(1)

with νL as the central laser frequency and ΔνL as the spectral width assumed to be proportional to νL. Typical values for the laser fluence and the spectral width were taken as15,16 Φ ≈ 1029 m−2 s−1 and ΔνL/νL = 0.5−1.5%. We neglect here the possible nonuniform spatial distribution of the laser beam, all molecules seeing the same laser intensity. A quartic force field is used to model the potential energy surface of the molecule, which through a second-order perturbative Dunham expansion yields a quadratic expression for the vibrational energy E(n) as a function of the quantum numbers n = {nk} E (n ) =

⎛

∑ ℏωk⎝nk + ⎜

k

1 ⎞⎟ + 2⎠

⎛

∑ χk S ⎝nk + ⎜

k≤S

1 ⎞⎟⎜⎛ 1⎞ nS + ⎟ ⎠ ⎝ 2 2⎠

σa/s(ν , E) =

1 H (E )

(2)

∑ ∑ σ(nk → nk ± 1) MC steps

k

× δ(hν − ΔEa/s)

This expression includes all harmonic frequencies νhk as well as mode anharmonicities and couplings, χkS . From the expression of E(n), all transition energies associated with absorption (nk → nk + 1) or emission (nk → nk − 1) events are readily obtained. Note that these transition energies, ΔEa/s(nk → nk ± 1), depend on the vibrational population n via anharmonicities, thereby causing some natural spectral broadening. This discrete model for the internal state of the molecule is computationally convenient because its statistical properties can be accurately determined by Monte Carlo sampling.17 The physical evolution of a molecule exposed to the FEL macropulse is simulated as follows. At time t = 0, a vibrational state n is randomly drawn from the thermal equilibrium population at fixed temperature Teq. As long as the time counter does not exceed tpulse, the molecule interacts with the laser and can undergo multiple photon absorptions as well as stimulated emission and possibly dissociation if the internal energy exceeds the dissociation threshold D. All of these events have to be characterized in terms of their respective rate constants, namely, ka, ks, and kd. Between any two events, the internal energy is constant. Furthermore, because the absorption and emission time scales both take place over more than nanoseconds under experimental conditions, intramolecular vibrational redistribution (IVR) can be assumed as sufficiently fast for all energy states to be equiprobable. This assumption is further justified in practice by the micropulse structure of the macropulse, under which the system evolves for

(3)

where H(E) is the number of entries with fixed internal energy. Rotational broadening at temperature Teq was also incorporated by convoluting these purely vibrational cross sections with a Gaussian profile with full width at half-maximum of 4(BkBTeq)1/2, with B the rotational constant and kB the Boltzmann constant. Spontaneous emission is treated similarly as stimulated emission but without any dependence on the laser fluence. Although lying beyond the experimental resolution of mass spectrometry,11 hydrogen emission was the only dissociation channel considered in the present study. The dissociation constant kd(E) was calculated via phase space theory in its orbiting transition state version but using here the anharmonic densities of states for both the parent and fragment obtained from Monte Carlo sampling 17,19 instead of harmonic estimates.20 The kMC trajectories are propagated until dissociation occurs or when the internal energy has dropped below D due to radiative cooling. They are then repeated a number of times 5 , and the number of trajectories 5d having dissociated either during or after exposure to the macropulse is calculated. The IRMPD intensity R(νL) is then estimated as ⎡ Vprobe 5 ⎤ × d⎥ R(νL) = −ln⎢1 − Vion 5⎦ ⎣ 3955

(4)



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where Vprobe/Vion is the fraction of the ion cloud irradiated by the laser taken here as 0.5. Finally, the IRMPD spectrum is obtained by varying the laser frequency νL.

The long time cooling relaxation, after exposure to the laser pulse, plays a crucial role in the finally measured IRMPD intensities. Under the conditions in Figure 2b, the dissociations contribute by only 2% to the band near 750 cm−1. This is the consequence that heating by the laser remains moderate and leads to slow statistical dissociation. The amount of energy deposited by the pulse also explains why some absorption bands are absent from the action spectrum. In the inset of Figure 2a, the average internal energy ⟨E⟩ at time tpulse exceeds the dissociation energy only near three spectral regions, which all appear in the IRMPD spectrum. For the most intense band (ω = 1205 cm−1), the average dissociation time is about 14 μs when Φ0 = 1.5 × 1029 m−2·s−1 and ΔνL = 1%. This value is much longer than the macropulse duration, in agreement with experimental results.10 In contrast, the band near 1400 cm−1 is active enough to heat the system up to 3.5 eV above the zero-point energy, but this value lies below D. Varying the laser intensity is most effective for such emerging bands that become observable once ⟨E⟩ exceeds D. This explains in particular the strong increase of the three least intense bands of Figure 2b upon raising the intensity by 33% to the conditions of Figure 2a. Another consequence is to bring the intensity of the two bands near 1200 cm−1 closer to each other. This nonlinear scaling of relative intensities found in Figure 2a−c is thus largely due to the internal energy lying below or above the dissociation threshold. In the absence of a continuous spatial beam profile, and as noticed earlier,21−23 this behavior would lead to a discontinuous dependence of the pulse energy needed to induce dissociation. Increasing the laser spectral width at fixed fluence has a minor effect on IRMPD intensities but is clearly responsible for a broadening of the bands, with the double-peak structure near 1200 cm−1 being lost as ΔνL/νL reaches 1.5% in Figure 2c. We now turn to vibrational shifts and discuss the role of anharmonicities on the IRMPD spectrum. Scaling the harmonic spectrum by an appropriate factor would of course bring the bands much closer to experiment, but we wish to avoid using any such empirical procedure here. Figure 3 highlights the most intense bands near 750 and 1200 cm−1 and compares the action spectrum with the linear absorption spectrum calculated at T = 300 K with a full account of anharmonicities. This latter spectrum was obtained from the quartic force field using the computational strategy described in ref 18, again after convolution by a Gaussian with fwhm of 4(BkBT)1/2 in order to account for rotational broadening. Intrinsic anharmonicities significantly shift the absorption bands by about 20 and 40− 50 cm−1 for the two spectral ranges, respectively. However, the calculated IRMPD spectra exhibit some extra red shift by 11 cm−1 for the lowest-frequency band and 17 cm−1 for the band at 1205 cm−1. The IRMPD experimental bands,11 also indicated in Figure 3, agree with the calculated IRMPD bands within a few wavenumbers, much better than that with the anharmonic absorption spectrum. The agreement is particularly good for the softest mode, but even for the band at 1205 cm−1, our results satisfactorily reproduce the additional red shift found by Oomens et al.11 in IRMPD experiments with respect to alternative measurements of IR absorption in matrix12 or argon tagging predissociation spectra.13 Conversely, the remaining band at 1180 cm−1 appears poorly shifted with respect to the absorption reference. In order to interpret these vibrational shifts, we consider how the average residence time ⟨τ⟩(E) changes with internal energy E as the laser frequency νL is varied, given that E grows essentially monotonically with time

3. APPLICATION TO THE NAPHTHALENE CATION The method was applied to cationic naphthalene, for which all necessary ingredients were obtained from density functional calculations using the hybrid B3LYP functional and the basis set 6-31+G*. Note that most of the computational cost goes into the determination of the anharmonic coefficients χkS . For this system, we found D = 4.8 eV and B = 0.0349 cm−1. The energy considered for the histograms ranged between the ground vibrational state up to 10 eV above, with a resolution of 100 cm−1. The spectral resolution was taken as 0.4 cm−1, and the initial conditions before exposure to the beam were sampled at Teq = 300 K. Three IRMPD spectra calculated with typical FEL parameters in the experimental range of 700−1700 cm−1 are shown in Figure 2. The absorption spectrum generally shows a

Figure 2. (a−c) Calculated IRMPD spectra of C10H+8 for various FEL parameters. In (a), the average internal energy at the end of the macropulse is represented as an inset for cations that have not dissociated and as a function of the central laser frequency. Panel (d) shows the harmonic absorption lines with unscaled frequencies. Laser fluxes Φ0 are expressed in m−2·s−1.

greater number of active bands with respect to the calculated IRMPD spectra, with only three main bands in the latter case near 750, 1185, and 1205 cm−1 and a less intense band near 1510 cm−1. Besides some systematic red shift, the line intensities show the most striking difference with respect to the harmonic absorption values. The intensity ratio between the two most intense bands near 750 and 1205 cm−1 is thus found to be about 3.2 in the IRMPD case with parameters from Figure 2b and only 2 in the harmonic absorption case. We shall come back to this issue later. 3956



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absorption bands. This mixing complicates the interpretation of IRMPD intensities and also incidentally explains why the band at 1185 cm−1 is more sensitive to the laser intensity than the band at 1220 cm−1, as suggested by Figure 2. The band at 1185 cm−1 is thus artificially enhanced due to the presence of a nearby band on the higher-energy side, a feature that has been noticed previously.10 The model also sheds some light on the influence of the FEL parameters on the calculated IRMPD spectrum. Varying either the fluence Φ0 or the spectral width ΔνL/νL reveals interesting features that are amenable to experimental comparison, and we show the corresponding IRMPD spectra in Figure 4 for the out-of-plane bending mode near 750 cm−1.

Figure 3. IRMPD spectrum (black curve) for FEL parameters of Φ0 = 1.5 × 1029 m−2·s−1 and ΔνL/νL = 1% and anharmonic linear absorption spectrum (red curve) calculated in the spectral ranges of (a) 680−780 and (b) 1120−1240 cm−1. The insets show the variations of the residence time τ as a function of internal energy E for two different laser frequencies, located by vertical arrows. The vertical dashed green lines highlight the experimental results from ref 11. In panel (b), the orange curve shows the absorption spectrum at internal energies shifted by +3 eV with respect to the 300 K thermal distribution.

Figure 4. Dependence of the calculated IRMPD spectrum on (a) the laser fluence Φ0 (in units of 1029 m−2·s−1) and (b) the relative spectral width ΔνL/νL.

As expected, the IRMPD intensity increases with the photon flux, but in a rather nonlinear fashion. The bandwidth increases linearly with Φ0 but does not shift by more than 1 cm−1 in the range presently studied. As the spectral width changes from 0.5 to 1.5%, minor increases in the bandwidth and the intensity are seen, together with a small red shift. That the IRMPD frequency does not vary much with laser fluence is consistent with experimental results,10 and it also provides some justification to our neglect of the spatial distribution of the laser beam, which could only affect the relative band intensities without producing significant shifts. We finally discuss the extent to which the IRMPD intensity measured at the maximum, Rmax, can be compared to the absorption intensity I given by the quantum chemical calculation. More precisely, we have investigated the variations of the IRMPD spectrum near two highly active bands with harmonic frequencies ω = 778.4 cm−1 (I = 107.0 km·mol−1) and ω = 1559.4 cm−1 (I = 85.9 km·mol−1). The intensity ratio between these bands is not well reproduced by the IRMPD spectra of Figure 2b and d, and looking at the IRMPD efficiency Rmax as a function of the laser fluence Φ0 (Figure 5) shows distinct variations for the two modes. In particular, both the onset where Rmax starts increasing and the value at which saturation occurs (Rsat = ln 2) are quite different. In the regime where the IRMPD efficiency varies linearly with Φ 0 , approximately near the fluence Φ01/2 where Rmax(Φ01/2) ≃ Rsat/2, the slope α is expected to be proportional to the intensity I. We find I/α = 175.4 and 308.8 km·mol−1·m−2·s−1 for the two bands located at 778.4 and 1559.4 cm−1, respectively. If we include the contribution of a nearby, also

during exposure to the macropulse. In the inset of Figure 3a, ⟨τ⟩ is shown for two laser frequencies chosen at the center of the theoretical absorption band and IRMPD absorption band, respectively. In the first situation, the residence time is initially very short because the molecule efficiently absorbs photons. However, this leads to some heating and a concomitant red shift of the absorption band due to anharmonicities. This drift of the absorption band leads to a smaller absorption cross section and is manifested by the increase in ⟨τ⟩ with E. In contrast, if the laser frequency is set to 751 cm−1 instead of 762 cm−1, ⟨τ⟩ is initially high due to the offset with respect to the absorption maximum, but at some stage, the system has become sufficiently heated for its red-shifted absorption band to approach the laser frequency, and the heating efficiency is higher. If νL is finally set further to the red, then the initial offset to absorption is too high, the system does not absorb enough photons for the absorption band to shift to the red sufficiently to match the laser frequency, and the IRMPD intensity remains low. A similar analysis in the higher spectral range gives results represented in the inset of Figure 3b. The initial residence time remains initially high because absorption is not very active. However, as E exceeds about 3 eV, the higher-frequency band initially centered at 1220 cm−1 displays a further red shift of approximately 15 cm−1 due to anharmonicities. The molecule can thus absorb more photons from this shifted band, ⟨τ⟩ drops, and the IRMPD band profile is a mixture of the two 3957



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can alter the vibrational spectrum, with red shifts of the bands originating from the continuous heating in multiphoton absorption and nontrivial intensity ratios in the case of partly overlapping bands. Such effects should be general and point to the key role of anharmonic couplings that allow the excitation energy to flow between modes and to relax by photon emission. This work could be extended to further include other relaxation pathways such as multiple dissociation channels competing with each other or isomerization. In particular, the presence of several isomers that are connected through known transition states should impact the IRMPD spectrum differently depending on whether these isomers were present from the beginning in the equilibrium sample or became available as the result of laser heating from the global-minimum geometry. Protonated aromatic hydrocarbons, which have been proposed as candidates to explain the unidentified infrared bands in the interstellar medium,7,25,26 should be good test cases in this respect as their isomers exhibit contrasted spectral signatures.24

Figure 5. Maximum IRMPD efficiency Rmax as a function of maximum laser fluence Φ0 for two harmonic vibrational bands located at 778.4 and 1559.4 cm−1.

■

active band with a harmonic frequency of 1578.9 cm−1 (I = 28.2 km·mol−1) to the IRMPD peak, the arithmetically averaged I/α is now equal to 203.7 km·mol−1·m−2·s−1. The residual discrepancy can be explained by the dynamical shift of the bands arising from the heating of the system under exposure to the FEL. The deviation of the IRMPD band relative to the fundamental frequency, β = [ν(f) − νIRMPD]/ν(f), was calculated for a selection of six well-isolated modes for which I/α was also evaluated at the laser spectral width of 0.1%. The results, given in Table 1, indicate that I/α is almost constant only when the dynamical shift β is small. It is then clear that relative intensities measured in IRMPD experiments do not faithfully mimic the calculated absorption intensities when anharmonicities are amplified by laser heating.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address ¶

M.B.: Département de Chimie, UMR Pasteur, Ecole Normale Supérieure, 24 rue Lhomond, 75231 Paris Cedex 05, France. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS ̂ for useful discussions. The authors wish to thank Dr. P. Maitre The LUMAT federation (FR LUMAT 2764) is acknowledged for providing computational resources with the facility cluster GMPCS. This work has been supported by the ANR through the project “Gas-phase PAH research for the interstellar medium” (ANR-10-BLAN-0501-GASPARIM).

4. CONCLUSIONS The IRMPD technique has become increasingly powerful and reliable for structural assignment of gas-phase compounds. However, its requirement of a thorough theoretical comparison with computed structures and the presence of phenomenological adjustment factors in the commonly used theories prevent this technique from being used on a routine basis. The present work was aimed at addressing the fundamental issue of the inherent differences between action and absorption spectra by modeling the entire action spectrum of an experimentally realistic molecule. Contrary to earlier approaches, we have tried to avoid any empiricism in our modeling by rooting it on ab initio ingredients and taking into account individual photon absorption and emission events and statistical relaxation, including carefully evaluated dissociation rates. Our application to the naphthalene cation has shown how the specific dynamics of the system exposed to the laser beam

■

REFERENCES

(1) Brodbelt, J. S.; Wilson, J. J. Infrared Multiphoton Dissociation in Quadrupole Ion Traps. Mass Spectrom. Rev. 2009, 28, 390−424. (2) Eyler, J. R. Infrared Multiple Photon Dissociation Spectroscopy of Ions in Penning Traps. Mass Spectrom. Rev. 2009, 28, 448−467. (3) Polfer, N. C.; Oomens, J. Vibrational Spectroscopy of Bare and Solvated Ionic Complexes of Biological Relevance. Mass Spectrom. Rev. 2009, 28, 468−494. (4) Fridgen, T. D. Infrared Consequence Spectroscopy of Gaseous Protonated and Metal Ion Cationized Complexes. Mass Spectrom. Rev. 2009, 28, 586−607. (5) Scott, A. P.; Radom, L. J. Harmonic Vibrational Frequencies: An Evaluation of Hartree−Fock, Moller−Plesset, Quadratic Configuration Interaction, Density Functional Theory, and Semiempirical Scale Factors. J. Phys. Chem. 1996, 100, 16502−16516.

Table 1. Harmonic (ν(h)) and Fundamental (ν(f)) Frequencies of Selected IR-Active Bands, Their Intensity I, Laser Fluence Φ01/2 at Half Saturation, Slope α of Rmax versus Φ0 at Φ0 = Φ01/2, Ratio I/α, and Relative Shift β of the IRMPD Line Relative to the Fundamental Frequency ν(h) (cm−1)

ν(f) (cm−1)

I (km·mol−1)

Φ01/2 (1029 m−2·s−1)

α (10−29 m2·s)

I/α (1029 km·mol−1·m−2·s−1)

β (%)

157.2 609.0 778.4 1042.1 1127.5 1310.8

156.2 602.8 764.6 1030.2 1110.0 1287.8

2.8 8.9 107.0 13.8 5.0 9.5

124.7 12.7 1.7 9.4 27.8 14.0

0.01 0.09 0.61 0.12 0.04 0.07

280 99 175 115 125 136

2.76 1.04 1.66 0.40 1.38 1.24

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(6) Banisaukas, J.; Szczepanski, J.; Eyler, J.; Vala, M.; Hirata, S.; Head-Gordon, M.; Oomens, J.; Meijer, G.; von Helden, G. Vibrational and Electronic Spectroscopy of Acenaphthylene and Its Cation. J. Phys. Chem. A 2003, 107, 782−793. (7) Lorenz, U. J.; Solcà, N.; Lemaire, J.; Maître, P.; Dopfer, O. Infrared Spectra of Isolated Protonated Polycyclic Aromatic Hydrocarbons: Protonated Naphthalene. Angew. Chem., Int. Ed. 2007, 46, 6714−6716. (8) Atkins, C. G.; Rajabi, K.; Gillis, E. A. L.; Fridgen, T. D. Infrared Multiple Photon Dissociation Spectra of Proton- And Sodium IonBound Glycine Dimers in the N−H and O−H Stretching Region. J. Phys. Chem. A 2008, 112, 10220−10225. (9) Oomens, J.; Tielens, A. G. G. M.; Sartakov, B. G.; von Helden, G.; Meijer, G. Laboratory Infrared Spectroscopy of Cationic Polycyclic Aromatic Hydrocarbon Molecules. Astrophys. J. 2003, 591, 968−985. (10) Oomens, J.; Sartakov, B. G.; Meijer, G.; von Helden, G. GasPhase Infrared Multiple Photon Dissociation Spectroscopy of MassSelected Molecular Ions. Int. J. Mass Spectrom. 2006, 254, 1−19. (11) Oomens, J.; Van Roij, A. J. A.; Meijer, G.; von Helden, G. GasPhase Infrared Photodissociation Spectroscopy of Cationic Polyaromatic Hydrocarbons. Astrophys. J. 2000, 542, 404−410. (12) Hudgins, D. M.; Sandford, S. A.; Allamandola, L. J. InfraredSpectroscopy of Polycyclic Aromatic Hydrocarbon Cations. 1. MatrixIsolated Naphthalene and Perdeuterated Naphthalene. J. Phys. Chem. 1994, 98, 4243−4253. (13) Piest, J. A.; von Helden, G.; Meijer, G. Infrared Spectroscopy of Jet-Cooled Cationic Polyaromatic Hydrocarbons: Naphthalene. Astrophys. J. 1999, 520, L75−L78. (14) Gillett, F. C.; Forrest, W. J.; Merrill, K. M. 8−13-Micron Spectra of NGC 7027, BD +30 3639, and NGC 6572. Astrophys. J. 1973, 183, 87−93. (15) Oepts, D.; van der Meer, A. F. G.; van Amersfoort, P. W. The Free-Electron-Laser User Facility FELIX. Infrared Phys. Technol. 1995, 36, 297−308. (16) Lemaire, J.; Boissel, P.; Heninger, M.; Mauclaire, G.; Bellec, G.; Mestdagh, H.; Simon, A.; Le Caer, S.; Ortega, J. M.; Glotin, F.; Maître, P. Gas Phase Infrared Spectroscopy of Selectively Prepared Ions. Phys. Rev. Lett. 2002, 89, 273002. (17) Basire, M.; Parneix, P.; Calvo, F. Quantum Anharmonic Densities of States Using the Wang−Landau Method. J. Chem. Phys. 2008, 129, 0811019. (18) Basire, M.; Parneix, P.; Calvo, F.; Pino, T.; Bréchignac, Ph. Temperature and Anharmonic Effects on the Infrared Absorption Spectrum from a Quantum Statistical Approach: Application to Naphthalene. J. Phys. Chem. A 2009, 113, 6947−6954. (19) Calvo, F.; Basire, M.; Parneix, P. Efficiency of Monte Carlo Methods for Densities and Sums of Rovibrational States. Chem. Phys. Lett. 2010, 496, 196−200. (20) Pino, T.; Parneix, P.; Calvo, F.; Bréchignac, Ph. Theoretical Rates for the Emission of Atomic Hydrogen from a Naphthalene Cation. J. Phys. Chem. A 2007, 111, 4456−4463. (21) MacAleese, L.; Maître, P. Infrared Spectroscopy of Organometallic in the Gas Phase: From Model to Real World Complexes. Mass Spectrom. Rev. 2007, 26, 583−605. (22) Simon, A.; Joblin, C.; Polfer, N.; Oomens, J. Infrared Spectroscopy of Complexes [XFeC24H12]+ (X = C5H5, C5(CH3)5) in the Gas Phase: Experimental and Computational Studies of Astrophysical Interest. J. Phys. Chem. A 2008, 112, 8551−8560. (23) Oomens, J.; Sartakov, B. G.; Tielens, A. G. G. M.; Meijer, G.; von Helden, G. Gas-Phase Infrared Spectrum of the Coronene Cation. Astrophys. J. 2001, 560, L99−L103. (24) Calvo, F.; Basire, M.; Parneix, P. Temperature Effects on the Rovibrational Spectra of Pyrene-Based PAHs. J. Phys. Chem. A 2011, 115, 8845−8854. (25) Ricks, A. M.; Douberly, G. E.; Duncan, M. A. The Infrared Spectrum of Protonated Naphthalene and Its Relevance for the Unidentified Infrared Bands. Astrophys. J. 2009, 702, 301−306.

(26) Ricca, A.; Bauschlicher, C. W.; Allamandola, L. J. Protonated Polycyclic Aromatic Hydrocarbons Revisited. Astrophys. J. 2011, 727, 128.

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Accurate Modeling of Infrared Multiple Photon Dissociation Spectra

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