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Femtosecond Excited State Dynamics of Size Selected Neutral Molecular Clusters Raul Montero, Iker León, Jose A. Fernandez, and Asier Longarte J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b00997 • Publication Date (Web): 07 Jul 2016 Downloaded from http://pubs.acs.org on July 7, 2016

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The Journal of Physical Chemistry Letters

Femtosecond Excited State Dynamics of Size Selected Neutral Molecular Clusters by Raúl Montero,1 Iker León,2 José A. Fernández,2 and Asier Longarte*,2 1 SGIker Laser Facility, UPV/EHU. Sarriena, s/n, 48940 Leioa, Spain 2 Departamento de Química Física. Universidad del País Vasco (UPV/EHU), Apart. 644, 48080 Bilbao, Spain

Author to whom correspondence should be addressed: Asier Longarte Departamento de Química-Física Facultad de Ciencia y Tecnología. Universidad del País Vasco (UPV/EHU) Apart. 644, 48080 Bilbao, Spain e-mail: [email protected] Phone ++34 946018086 Fax: ++ 34 946013500

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Abstract The work describes a novel experimental approach to track the relaxation dynamics of an electronically excited distribution of neutral molecular clusters formed in a supersonic expansion, by pump-probe femtosecond ionization. The introduced method overcomes fragmentation issues, permitting to extract the dynamical signature of a particular cluster from each mass channel, by associating it to an IR transition of the targeted structure. We have applied the technique to study the non-adiabatic relaxation of pyrrole homoclusters. The results obtained exciting at 243 nm, near the origin of the bare pyrrole electronic absorption, allow us to identify the dynamical signature of the dimer (Py)2, which exhibits a distinctive lifetime of τ1~270 fs, considerably longer than the decays recorded for the monomer and bigger size clusters (Py)n>2. A possible relationship between the measured lifetime and the clusters geometries is tentatively discussed.

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Time and frequency domain spectroscopic studies on size controlled noncovalently bonded molecular aggregates permit to explore the physical properties of the targeted molecules, under the influence of specific moleculemolecule interactions (van der Waals forces and hydrogen bonds). Because of the direct structure-properties relationship these species yield, the role of individual intermolecular interactions can be effectively addressed. This ability has made of molecular clusters one of the most promising tools in chemical physics, enabling to fill the gap between the isolated gas-phase and the intrinsically complicated condensed or solid phases (see for instance Ref.1-3 and references therein). With the appearance of ultrashort pulse sources, femtosecond time-resolved studies of electronically excited molecular clusters, in particular of the form solute(solvent)n, were recognized as an efficient method to unravel the effect of solvation, on the photophysical and photochemical properties of relevant chromophoric molecules.4 Since then, numerous works have explored the ultrafast dynamics of aggregates formed between aromatic chromophores, such as pyrrole, indole or DNA basis, and protic solvents as water and ammonia.5,6 Nowadays, firmly grounded on the detailed information gained in the last years from gas phase photo-dynamical studies,7-10 an important effort is devoted to link the photochemistry/photophysics of isolated chromophores with their more complex behavior observed in solution.11,12 In this sense, the detailed experimental and theoretical information provided by molecular clusters, regarding the interaction with explicit molecules of the closest solvent shell, appears as one of the keys to solve the problem.6,13,14 The

photoinduced

dynamics

of

clusters

containing

multiple

aromatic

chromophores, as the (pyrrole)n complexes targeted here, can be considered



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attending to its scope and implications, another well differentiated hot topic. The isolated multi-chromophoric aggregates held by intermolecular forces seem an ideal environment to unravel the mechanisms of photoinduced charge (proton or electron) transfer processes,5,15-23 in particular those involving electronic delocalization.24,25 Since the methods to generate molecular clusters offer a limited control over the size distribution, time-resolved measurements able to extract the dynamics of specific size clusters, as those based in the detection of mass-selected ions, are required. However, the fragmentation processes that these weakly bound species undergo notably complicate the identification of the different species contributing to each mass-channel.5,26,27 Evaporation occurs when electronic states with dissociative character coordinates are prepared, or more generally, when the electronic excitation is transferred to the intermolecular vibrational modes. The fragmentation pattern can be highly intricate, as the dissociation can take place in the directly excited electronic state, in the relaxed lower electronic states, or alternatively, in the ion state after the interaction with the probe beam. Aiming to overcome the above explained difficulties of studying the relaxation dynamics of neutral clusters, herein we introduce a methodology that permits to disentangle the transient signal belonging to a particular size cluster, from the overall dynamics recorded in a mass channel, by associating it to a characteristic IR absorption of the targeted complex. The basics of the experiment, which builds

upon frequency resolved double

resonance

techniques28-30 and combines ns and fs pulses31,32 to gain time and frequency resolution, is graphically explained in Scheme 1. An IR ns pulse, tuned to the


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vibrational transition of the cluster of interest, interacts with the supersonic expansion at a fixed time prior to the fs pump/probe pulses that will prepare and track the excited state dynamics. The temporal evolution of the selected mass channels is recorded by monitoring the ion signal at different pump/probe delays, with the IR laser alternatively on and off. The selective excitation of an IR active vibration of the cluster, usually N-H or O-H stretchings are the modes of choice, will lead, in general, to dissociation, as the deposited energy exceeds the intermolecular interaction strength. Then, the dynamical signature of the IR excited cluster will be depleted from every channel where it was present, while at the same time, an enhancement on the dynamics of the product fragments will be observed. Therefore, by examining the changes induced by the IR beam in the appropriate mass channels, it is possible to extract the dynamics of the cluster selected by the IR excitation. An extended explanation on the experimental implementation of the technique can be found as Supporting Information (SI). To demonstrate the potential of the approach, we have conducted experiments on the system (Pyrrole)n. Attracted by the unusual photochemical properties of pyrroles, a considerable number of recent theoretical and experimental studies have aimed to unravel the non-radiative relaxation process that mediate the relaxation of the pyrrole ring,25,33-40 and to a lesser extent, the influence of solvents

on

them.41-44

The

dynamical

information

gained

on

the

photochemical/photophysical processes involving the lowest singlet excited state of pyrrole, which as a result of a π→Nitrogen 3s excitation acquires 1πσ* character along the N-H bond, has notably contributed to develop the actual understanding on the role of these type of “dark” transitions in the



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photochemistry of simple heteroaromatic molecules. It is assumed that at 243 nm the pyrrole monomer is barely electronically excited by vibronic coupling, yielding a short living 1A2 1πσ* state that leads to barrierless dissociation of the N-H bond in less than 20 fs.25,39,40 The transient showed in Figure 1 was collected in the pyrrole+ (Py+) mass channel by exciting at 243 nm and ionizing with three 800 nm photons (1+3’), after adjusting the conditions to avoid the occurrence of clustering processes (see SI). The Py+ time profile appears as a Gaussian function analogous to that resulting from a pump+probe non-resonant ionization cross-correlation function (see Ref. 38 for a detailed discussion on the matter). This observation has been explained before as the result of the small population promoted to an extremely short lived 1A2 1πσ* excited state.38,39 In the following, we will refer to this temporal behavior which is described by a Gaussian cross-correlation function convoluted with a much shorter exponential, as τ0. Figure 2a shows a mass spectrum recorded at expansion conditions that favor the formation of pyrrole homoclusters, and at zero delay between the 243 nm pump and the 800 nm probe. The higher detected mass corresponds to (Py)3+, while the ion masses distribution responds to the percentages: 100:7:0.2. Here it is important to note that the ionization potentials of pyrrole clusters are calculated to be slightly lower than that of the monomer (8.21 eV), since very likely the order of the multiphoton ionization drops from 1+3’ for the bare pyrrole, to 1+2’ for the clusters.25,45 Figure 2b was obtained by scanning the IR laser while registering the IRon-IRoff signal at the (Py)2+ channel. Three N-H stretch absorptions located at 3405, 3450 and 3521 cm-1 are readily observed. Based on the IR characterization of pyrrole clusters by Matsumoto et al.46 and


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Dauster et al.,47 the bands at 3450 and 3521 cm-1 can be unambiguously assigned to the dimer, however that at 3405 cm-1, overlaps with absorptions attributed to the trimer (3392 cm-1), the tetramer (3382 cm-1), the pentamer (3406 cm-1) and even higher order clusters. Consequently, although the trimer is probably the main contributor to the 3405 cm-1 band, we can not discard the presence of bigger species. Nevertheless, it is important to note that the IR spectrum shown was recorded at the (Py)2+, indicating that the mass intensity distribution measured is highly affected by fragmentation processes which, as the delay time between the pump and probe pulses is set to zero, necessarily occur in the ionization step. This fact nicely illustrates the already known paradigm of how the measured mass spectrum does not necessarily reflect the beam composition. The next step, once the composition of the beam was analyzed, was to isolate the excited state dynamics of the different pyrrole clusters present in the beam. In order to do so, we fixed the IR excitation at the different transitions observed in the spectrum, while recording the temporal evolution of the pyrrole dimer and monomer ion mass channels. In all cases the clusters were excited at 243 and probed with 800 nm radiation. Figure 3 shows the transients acquired in the Py+ and (Py)2+ channels after fixing the IR source at 3445 cm-1, along the band assigned to the H-bonded N-H stretching of the dimer. At these conditions, the transient resulting of the IRon-IRoff difference should provide the dynamical information of the (Py)2 contained in those channels, which for the dimer channel, yields the negative blue trace exhibited in panel 3a. This decay, fit by a τ0 component, analogous to the cross-correlation Gaussian function observed for the bare pyrrole, plus an additional τ1=270±19 fs lifetime, corresponds to the



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relaxation of the (Py)2 cluster, and its negative character comes from the reduction of the dimer signal after the fragmentation induced by the absorbed IR photon. We will return to briefly discuss on the implications of this signal. Panel 3b shows the transients recorded in the monomer channel. Conversely to what was observed for the dimer, the presence of the IR produces a very modest effect on the Py+ signal, indicating that the dimer contribution to the monomer mass-channel represents a small fraction of the total ion signal. However, after zooming into the difference ion signal (Fig. 3b), the dimer τ1 decay is noticeable at positive delay times, revealing the presence of some fragmented dimers. This contribution appears as a dip in the monomer signal (negative sign), as the absorption of the IR photon leads to the dimers dissociation. It is important to note that although the values of the τ1 lifetime extracted at the (Py)2+ and Py+ mass channels vary slightly they reflect the same observable: the excited state lifetime of the dimer. Figure 4 exhibits the transients recorded in the dimer (4a) and the monomer (4b) mass channels with the IR laser tuned at the 3400 cm-1 band, corresponding to the species (Py)n>2. The difference decay obtained in the dimer channel (blue trace in Fig. 4a) is composed of a negative Gaussian τ0 contribution, followed by the τ1=256±123 fs lifetime that oppositely, presents positive character. The observed enhancement of the dimer τ1 component is due to the new dimers formed by the IR excitation. However, a reduction of τ0 is obtained at the same conditions, when in principle, as this component is also present in the dimer, we should also expect an increment. This means that most of the τ0 component recorded at the (Py)2+ channel (in the absence of the IR radiation) was caused by bigger clusters fragmenting upon ionization. As these


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species are depleted from the beam by the IR, their contribution to the τ0 component is reduced. Consequently, at the (Py)2+ channel, two opposite going contributions to the τ0 component are promoted by the IR absorption: dimer increment vs (Py)n>2 depletion, being the later the dominant process. Accordingly, the (Py)n>2 clusters depleted by the IR absorption are moved to the monomer channel, causing the large increment of the τ0 component observed in Figure 4b. Although a detailed analysis of these data will require further experimental and theoretical work, several valuable preliminary ideas, regarding the (Py)n>2 species, can be drawn: i) the decay associated to the (Py)n>2 clusters, has essentially the form of a Gaussian function with no measurable lifetime, analogously to the decay observed for the monomer. Since the photoionization window is lost during the cross-correlation time, we can infer that either a very short living excited state is reached, or the ionization of these clusters occurs through a non-resonant 1+2’ ionization process. ii) the important increase of monomer ions detected by exciting the N-H stretch of (Py)n>2 clusters reveals that, although the experimental conditions were set to minimize molecular aggregation, the expansion is rich in these species. Additionally, the modest increment of dimers population compared to that of the monomers indicate that the (Py)n>2 clusters yield preferentially monomer fragments after IR excitation. iii) the dimers produced by IR-induced fragmentation of bigger species exhibit the same temporal behavior, than those directly formed in the expansion. From this type of dynamical data, relevant information on the geometries and fragmentation patterns of molecular clusters can potentially be retrieved.



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Finally, we would like to briefly comment on the peculiar relaxation pattern exhibited by the dimers. In addition to the τ0 component, an additional τ1~270 fs lifetime was extracted from the signal fit, indicating that a longer living excited state is involved in the relaxation of these clusters. This intriguing lifetime has been recently reported and also attributed to the presence of dimers, in the time resolved photoelectron measurements by Neville et al.25 According to their ab

initio calculations, the existence of a longer living state with charge transfer character between the pyrrole moieties of the dimer would be on the origin of the extended τ1~270 fs lifetime. Although the present study can not provide further insights on the relaxation process involved, it permits to unambiguously identify the dimer as the responsible cluster, and additionally, reveals the distinctive dynamics of this cluster from bigger size species. In line with the motivating results by Neville et al., we can raise some speculations regarding the overall dynamical behavior of the pyrrole clusters that are guiding the research running these days in our lab. The geometries of pyrrole homoclusters have been studied by spectroscopic and theoretical methods.46,48 The most stable calculated geometries, which agree with those detected in supersonic expansions, are shown in Scheme 1b. Interestingly, apart from the dimer, the clusters up to the pentamer adopt cyclic structures in which all the pyrrole molecules are equivalent. For these centrosymmetric structures, the electronic wavefunction is necessarily delocalized among the pyrrole units that form the supramolecular entity. This localized nature of the excitation that the dimer, contrarily to bigger species, exhibits, could be responsible of its characteristic dynamical behavior. Nevertheless, complementary experimental measurements and extended theoretical calculations, on the electronic structure of the (Py)n>2


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clusters are required before a reasonable picture of the outlined dynamics can be built. In summary, herein we introduce an experimental method that can notably contribute to the dynamical study of the relaxation processes of molecular clusters. In order to illustrate its capabilities, we have applied it to track the ultrafast excited state non-adiabatic dynamics of pyrrole homoclusters. The present experiments have permitted us to unravel the distinctive dynamical behavior of the pyrrole dimer, which after excitation at 243 nm, decays with a lifetime longer than the monomer and the bigger species formed in the expansion. The developed technique can be applied to any molecular cluster presenting a distinctive IR signature, as long as the IR absorption induces the partial evaporation of the cluster’s components. In addition to its application to the study of multi-chromophore aggregates, it could be particularly useful to time resolve photoinduced charge (proton and electron) and hydrogen transfer processes that occur in H-bonded clusters of the form solute(solvent)n.



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Scheme 1. a) Schematics of the experimental method applied to track the relaxation dynamics of the pyrrole clusters (see text for details). b) Most stable geometries calculated for the (Py)n=2-5 clusters.48


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Figure 1. Transient recorded in the Py+ mass, after adjusting the expansion conditions to avoid the formation of clusters, by exciting at 243 nm and probing with 800 nm radiation (1+3’). The red line corresponds to the Gaussian fit obtained.



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Figure 2. a) Mass spectrum collected at expansion conditions that favored the formation of dimers, at zero delay time between the 243 nm pump and the 800 nm probe. The presence of some contamination peak is indicated by an asterisk. b) IR spectrum recorded at the (Py)2+ mass channel (see text for details). The red and blue colored peaks correspond to the (Py)2 and (Py)n>2 species, respectively. The solid lines indicate the wavelengths at which the dynamics of the Py+ and (Py)2+ mass channels was tracked.


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Figure 3. Transients recorded at the (Py)2+ (a) and Py+ channels (b), with the IR nanosecond laser tuned at 3445 cm-1. The red and black dots are the transients recorded, with the IR ns laser on and off, respectively, by exciting at 243 nm and probing with 800 nm radiation. The blue dots and line correspond to the transient obtained by subtracting the IRon and IRoff decays and its multiexponential fit. The green and orange lines represent the individual temporal components of the fit. For panel b) the region around ∆t=0 shows an artifact due to the incomplete subtraction of the signals, and only the long τ1=337±87 fs is used to model the transient.



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Figure 4. Transients recorded at the (Py)2+ (a) and Py+ channels (b), with the IR nanosecond laser tuned at 3400 cm-1. The red and black dots are the transients recorded, with the ns IR laser on and off, respectively, by exciting at 243 nm and probing with 800 nm radiation. The blue dots and line correspond to the transient obtained by subtracting the IRon and IRoff signals and its multiexponential fit. The green and orange lines represent the individual temporal components of the fit.


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Acknowledgements We thank the support from the Spanish MINECO through the CTQ2015-68148C2-1-P grant. The work was also funded by the Basque Government through the “Ayudas para apoyar las actividades de grupos de investigación del sistema universitario vasco” program. I.L. thanks the MINECO for a postdoctoral fellowship. The experimental measurements were carried out at the SGIker Laser Service of the UPV/EHU. Supporting Information for Publication Extended details on the experimental method employed.



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(24) Schemmel, D.; Schütz, M. Molecular Aniline Clusters. Ii. The Low-Lying Electronic Excited States. J. Chem. Phys. 2010, 133, 134307. (25) Neville, S. P.; Kirkby, O. M.; Kaltsoyannis, N.; Worth, G. A.; Fielding, H. H. Identification of a New Electron-Transfer Relaxation Pathway in Photoexcited Pyrrole Dimers. Nat Commun 2016, 7, 11357. (26) Buck, U.; Huisken, F. Infrared Spectroscopy of Size-Selected Water and Methanol Clusters. Chem. Rev. 2000, 100, 3863-3890. (27) Peralta Conde, A.; Ovejas, V.; Montero, R.; Castaño, F.; Longarte, A. Influence of Solvation on the Indole Photophysics: Ultrafast Dynamics of Indole–Water Clusters. Chem. Phys. Lett. 2012, 530, 25-30. (28) Demtröeder, W. Laser Spectroscopy; Springer-Verlag: Berlin, 2008. (29) Zwier, T. S. Laser Spectroscopy of Jet-Cooled Biomolecules and Their Water-Containing Clusters: Water Bridges and Molecular Conformation. J Phys. Chem. A 2001, 105, 8827-8839. (30) Shubert, V. A.; Zwier, T. S. IR−IR−UV Hole-Burning: Conformation Specific IR Spectra in the Face of UV Spectral Overlap. J. Phys. Chem. A 2007, 111, 13283-13286. (31) Nosenko, Y.; Kunitski, M.; Thummel, R. P.; Kyrychenko, A.; Herbich, J.; Waluk, J.; Riehn, C.; Brutschy, B. Detection and Structural Characterization of Clusters with Ultrashort-Lived Electronically Excited States: IR Absorption Detected by Femtosecond Multiphoton Ionization. J. Am. Chem. Soc. 2006, 128, 10000-10001. (32) León, I.; Montero, R.; Longarte, A.; Fernández, J. A. IR Mass-Resolved Spectroscopy of Complexes without Chromophore: Cyclohexanol(H2O)n, n = 1–3 and Cyclohexanol Dimer. J. Chem. Phys. 2013, 139, 174312. (33) Lippert, H.; Ritze, H. -.; Hertel, I. V.; Radloff, W. Femtosecond TimeResolved Hydrogen-Atom Elimination from Photoexcited Pyrrole Molecules. ChemPhysChem 2004, 5, 1423-1427. (34) Cronin, B.; Nix, M. G. D.; Qadiri, R. H.; Ashfold, M. N. R. High Resolution Photofragment Translational Spectroscopy Studies of the Near Ultraviolet Photolysis of Pyrrole. Phys. Chem. Chem. Phys. 2004, 6, 5031-5041. (35) Barbatti, M.; Pittner, J.; Pederzoli, M.; Werner, U.; Mitrić, R.; BonačićKoutecký, V.; Lischka, H. Non-Adiabatic Dynamics of Pyrrole: Dependence of Deactivation Mechanisms on the Excitation Energy. Chem. Phys. 2010, 375, 26-34. (36) Roberts, G. M.; Williams, C. A.; Yu, H.; Chatterley, A. S.; Young, J. D.; Ullrich, S.; Stavros, V. G. Probing Ultrafast Dynamics in Photoexcited


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Pyrrole: Timescales for 1πσ* Mediated H-Atom Elimination. Faraday Discuss. 2013, 163, 95-116. (37) Neville, S. P.; Worth, G. A. A Reinterpretation of The Electronic Spectrum of Pyrrole: A Quantum Dynamics Study. J. Chem. Phys. 2014, 140, 034317. (38) Montero, R.; Ovejas, V.; Fernández-Fernández, M.; Peralta Conde, Á; Longarte, A. Revisiting the Relaxation Dynamics of Isolated Pyrrole. J. Chem. Phys. 2014, 141, 014303. (39) Wu, G.; Neville, S. P.; Schalk, O.; Sekikawa, T.; Ashfold, M. N. R.; Worth, G. A.; Stolow, A. Excited State Non-Adiabatic Dynamics of Pyrrole: A TimeResolved Photoelectron Spectroscopy and Quantum Dynamics Study. J. Chem. Phys. 2015, 142, 074302. (40) Makhov, D. V.; Saita, K.; Martinez, T. J.; Shalashilin, D. V. Ab Initio Multiple Cloning Simulations of Pyrrole Photodissociation: TKER Spectra and Velocity Map Imaging. Phys. Chem. Chem. Phys. 2015, 17, 3316-3325. (41) David, O.; Dedonder-Lardeux, C.; Jouvet, C.; Kang, H.; Martrenchard, S.; Ebata, T.; Sobolewski, A. L. Hydrogen Transfer in Excited Pyrrole– Ammonia Clusters. J. Chem. Phys. 2004, 120, 10101-10110. (42) Rubio-Lago, L.; Zaouris, D.; Sakellariou, Y.; Sofikitis, D.; Kitsopoulos, T. N.; Wang, F.; Yang, X.; Cronin, B.; Devine, A. L.; King, G. A.; Nix, M. G.; Ashfold, M. N. R.; Xantheas, S. S. Photofragment Slice Imaging Studies of Pyrrole and the XegPyrrole Cluster. J. Chem. Phys. 2007, 127, 064306. (43) Frank, I.; Damianos, K. Excited State Dynamics in Pyrrole–Water Clusters: First-Principles Simulation. Chem. Phys. 2008, 343, 347-352. (44) Rodríguez, J. D.; González, M. G.; Rubio-Lago, L.; Bañares, L. Photodissociation of Pyrrole-Ammonia Clusters Below 218 nm: Quenching of Statistical Decomposition Pathways Under Clustering Conditions. J. Chem. Phys. 2012, 137, 094305. (45) Profant, V.; Poterya, V.; Fárník, M.; Slavíček, P.; Buck, U. Fragmentation Dynamics of Size-Selected Pyrrole Clusters Prepared by Electron Impact Ionization: Forming a Solvated Dimer Ion Core. J. Phys. Chem. A 2007, 111, 12477-12486. (46) Matsumoto, Y.; Honma, K. NH Stretching Vibrations of Pyrrole Clusters Studied by Infrared Cavity Ringdown Spectroscopy. J. Chem. Phys. 2007, 127, 184310. (47) Dauster, I.; Rice, C. A.; Zielke, P.; Suhm, M. A. N-Hgπ Interactions in Pyrroles: Systematic Trends from the Vibrational Spectroscopy of Clusters. Phys. Chem. Chem. Phys. 2008, 10, 2827-2835.



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Scheme 1. a) Schematics of the experimental method applied to track the relaxation dynamics of the pyrrole clusters (see text for details). b) Most stable geometries calculated for the (Py)n=2-5 clusters.48 177x57mm (300 x 300 DPI)



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Figure 1. Transient recorded in the Py+ mass, after adjusting the expansion conditions to avoid the formation of clusters, by exciting at 243 nm and probing with 800 nm radiation (1+3’). The red line corresponds to the Gaussian fit obtained. 82x66mm (300 x 300 DPI)


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Figure 2. a) Mass spectrum collected at expansion conditions that favored the formation of dimers, at zero delay time between the 243 nm pump and the 800 nm probe. The presence of some contamination peak is indicated by an asterisk. b) IR spectrum recorded at the (Py)2+ mass channel (see text for details). The red and blue colored peaks correspond to the (Py)2 and (Py)n>2 species, respectively. The solid lines indicate the wavelengths at which the dynamics of the Py+ and (Py)2+ mass channels was tracked. 176x71mm (300 x 300 DPI)



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Figure 3. Transients recorded at the (Py)2+ (a) and Py+ channels (b), with the IR nanosecond laser tuned at 3445 cm-1. The red and black dots are the transients recorded, with the IR ns laser on and off, respectively, by exciting at 243 nm and probing with 800 nm radiation. The blue dots and line correspond to the transient obtained by subtracting the IRon and IRoff decays and its multi-exponential fit. The green and orange lines represent the individual temporal components of the fit. For panel b) the region around ∆t=0 shows an artifact due to the incomplete subtraction of the signals, and only the long τ1=337±87 fs is used to model the transient. 82x130mm (300 x 300 DPI)


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Figure 4. Transients recorded at the (Py)2+ (a) and Py+ channels (b), with the IR nanosecond laser tuned at 3400 cm-1. The red and black dots are the transients recorded, with the ns IR laser on and off, respectively, by exciting at 243 nm and probing with 800 nm radiation. The blue dots and line correspond to the transient obtained by subtracting the IRon and IRoff signals and its multi-exponential fit. The green and orange lines represent the individual temporal components of the fit. 82x129mm (300 x 300 DPI)


Femtosecond Excited State Dynamics of Size ... - ACS Publications

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