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Time-Resolved Dissociation Dynamics of Iodomethane Resulting from Rydberg and Valence Excitation Arne Baumann, Dimitrios Rompotis, Oliver Schepp, Marek Wieland, and Markus Drescher J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b01248 • Publication Date (Web): 10 May 2018 Downloaded from http://pubs.acs.org on May 11, 2018

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The Journal of Physical Chemistry

Time-resolved Dissociation Dynamics of Iodomethane Resulting from Rydberg and Valence Excitation Arne Baumann,∗,†,‡ Dimitrios Rompotis,¶ Oliver Schepp,† Marek Wieland,†,‡,§ and Markus Drescher∗,†,‡,§ †Institute for Experimental Physics, University Hamburg, Hamburg, Germany ‡The Hamburg Centre for Ultrafast Imaging - CUI, Hamburg, Germany ¶Deutsches Elektronen-Synchrotron - DESY, Hamburg, Germany §Center for Free-Electron Laser Science - CFEL, Hamburg, Germany E-mail: [email protected]; [email protected]

Abstract

abling intermediate states leading to the desired reaction products. 1–3 With photon energies in the vacuum ultraviolet spectral range a multitude of Rydberg excitations are accessible, which may act not only as mere spectators, but as gateway states facilitating ultrafast reaction dynamics. Very often they enable new reaction pathways 4,5 or efficient photodeactivation mechanisms of fundamental importance to nucleobases and proteins. 6,7 In this work the photodissociation dynamics of iodomethane, initiated by UV-excitation in the first absorption band and by single VUVphoton excitation of the 6p (2 E3/2 ) Rydberg state are investigated in a single experiment. Excitation of the A-band (220-350 nm) has received significant attention as it constitutes a prototypical system to study sub-100-fs dissociation dynamics, which are influenced by a nonadiabatic crossing. Many experiments 8–22 with a variety of excitation and detection schemes, as well as theoretical studies 16,23–25 have been conducted to elucidate the importance of different reaction channels mediated by initial population of different vibrational and electronic states. However, with exception of the lowlying Rydberg states in the B-band, 26–33 timeresolved studies of the Rydberg state reaction dynamics have so far only been conducted by

Rydberg excitations in the vacuum ultraviolet spectral range may open up molecular photoreaction pathways not accessible from lower-lying valence states. Here, single-shot UV/VUV pump-probe spectroscopy was used to study the photodissociation dynamics of iodomethane after 268 nm excitation in the A band and excitation of the 6p (2 E3/2 ) Rydberg state at 161 nm. By combining weak-field VUV singlephoton ionization with sub-10-fs temporal resolution and the superior statistical accuracy of the single-shot technique, sub-30 fs wave packet dynamics upon excitation in the A-band by a UV pump pulse were disclosed. Population transfer from the Rydberg state to the 2 1 A1 valence state leading to 100-fs dissociation dynamics was observed by utilizing the same methodology in a VUV-pump/UV-probe scheme.

Introduction Ultrafast photoinduced reaction dynamics are often governed by the interplay between multiple electronic states connected by non-adiabatic crossings, allowing the rapid transfer of population from an initially excited state to en-

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multi-photon excitation 5,17 and their potential to enable yet undiscovered dynamics is widely unexplored. In contrast to two-photon induced dynamics, reactions initiated by single-photon absorption cannot only be realized in ultrafast laser laboratories and provide information about photochemical mechanisms that may occur at the upper layers of Earth’s atmosphere. Iodomethane is not only a prototypical system to study femtosecond dissociation dynamics, but its decomposition is relevant in environmental chemistry. 34 The presented study is tailored to investigate molecular decomposition mediated by Rydberg state excitation in a largely unexplored spectral domain. For these means, a combination of intense 16fs, 161-nm (7.7-eV) 35 and 21-fs, 268-nm (4.6eV) pulses is utilized to perform a single-shot VUV/UV pump-probe experiment. 36 The ν1 vibrational state of the 6p (2 E3/2 ) Rydberg state is excited by single VUV-photon absorption 37 and acts as a gateway state to initiate dissociation along the H3 C−I bond, which is then probed by one or two UV-photons. Simultaneously, the A band dissociation dynamics are accessed, when pump- and probe pulses are interchanged and the UV pulse arrives first.

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Odd harmonics of the fundamental 805-nm radiation provided by a commercial Ti:Sa laser system (15 mJ pulse energy, 35 fs FWHM pulse duration, 25 Hz repetition rate) are created in Argon utilizing a loose-focusing geometry (f = 5 m). The wavefront of the harmonic beam was split into two halves by a Si wedge cut at Brewster’s angle for 800 nm, suppressing the fundamental pulse and creating a pair of spatially separated harmonic partial beams. Each harmonic beam followed a separate symmetric beam path, encountering two spherical mirrors (R = 300 mm), coated to spectrally purify the 268-nm or the 161-nm beam respectively, before they were brought to spatial and temporal overlap in a common focus. A pulsed gas valve (Parker Series 9 solenoid valve) provided the gaseous target in the common focal region. The photoionization products were imaged with a mass/charge state selective ion imaging spectrometer providing micrometer resolution, similar to the one described in Ref. 38,39. The temporal delay between the counter-propagating pulses is mapped onto a spatial coordinate, enabling single-shot cross correlation and pumpprobe measurements. The energy provided by the 268-nm and 161-nm radiation was reduced to less than 500 nJ to eliminate contributions to the parent ion signal from multi-photon ionization by each individual pulse. Iodomethane was supplied from the vapor of the pure liquid (Sigma Aldrich, 99 %). The sub-21-fs harmonic pulses grant the required temporal resolution to study dynamics in the sub-10-fs range and follow the processes steering the reaction dynamics directly after the excitation. 40,41 While resonance-enhanced multiphoton ionization (REMPI) schemes allow interrogating individual dissociation path ways, the excitation scheme utilized here can be applied readily to a wide range of molecules. The weak-field VUV-probe avoids light-induced effects on the molecular potential energy landscape. 42,43 When multi-photon IR probe ionization is used instead, great care has to be taken to evaluate the relevance and impact of these effects. 18 This influence is most pronounced in the vicinity of the delay time origin and within the probe pulse duration, as the molecular sys-

Experimental methods Our previously demonstrated high-throughput single-shot pump-probe technique relies on mass/charge-selective imaging of the charged particle distribution created by counterpropagating UV/VUV pulses by mapping the temporal delay between the pulses onto a spatial coordinate. It allows to collect thousands of complete pump-probe data sets within few minutes enabling experimental results providing statistically highly significant data in a short acquisition time, thus effectively suppressing drifts of the experimental conditions. Details about the generation of sub-20-fs VUV pulses are presented in Ref. 35. The single-shot pump-probe technique is described in detail in Ref. 36. Here only a short description will be provided.


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tem may be effectively dressed by the strong IR probe field.

proximated as an exponential function) for an initial C-I bond length of 2.2 Å as indicated in Fig. 1 a. The delay-dependent ion yield is retrieved from the spatial ion distribution (Fig. 2 a) acquired with the mass/charge-selective imaging ion spectrometer. The vertical coordinate in the image is averaged and the respective delay between the counter-propagating UV and VUV pulses is mapped along their common propagation axis on the horizontal coordinate. The arrows indicate the delay convention used in the discussion of the presented results: Positive delays refer to the UV pulse arriving early and the VUV pulse acting as the probe pulse and viceversa for negative delay times. In this detection scheme the delay-dependent ion yield for parent and fragment ions is retrieved for all delay times on a single-shot basis and the average over 3000 complete delay data sets is presented in Figure 2 a. The finite Gaussian instrument response function (IRF) given by the cross correlation of UV and VUV pulses of 26.1 ± 1.7 fs was measured experimentally in the same experimental run (see Supporting Information for details about the determination of the IRF). The delay-dependence of the CH3 I parent ion yield (Fig. 2 b) is modeled by the convolution of the IRF with a bi-exponential decay, with the time constants τ (3 Q0 ) = 98.2 ± 1.9 fs and τ (1 Q1 ) = 28.5 ± 2.8 fs, corresponding to the population decay times of the respective states. Our finding for τ (3 Q0 ) is in excellent agreement with previously published time-resolved studies for the 3 Q0 channel, which state a reaction time of 94 ± 6 fs. 20 τ (1 Q1 ) is interpreted as the dura˜ 1 E3/2 ionization window in tion of the 1 Q1 → X very good agreement with the classical trajectory calculations taking into account the ionization window restrictions in the case of our probe wavelength. The τ (3 Q0 ) time constant directly relates to the molecular dissociation time along the CH3 −I reaction coordinate followed to the asymptotic PES region. For the discussion of the delay-dependent I+ and CH3+ fragment yields different mechanisms need to be considered. 18 After preparation of the excited state (CH3 I∗ ) in the pump step charged fragments are primarily detected as a

Results & Discussion Dissociation after UV Valence Excitation We will first focus on the UV photodissociation via the A absorption band, and the potential energy surfaces (PES) enabling these reaction dynamics. Figure 1 shows a 1D cut along the H3 C−I bond coordinate, which in this case corresponds to the main reaction coordinate, through the relevant PES for CH3 I 44 and for CH3 I+ . 45,46 At a wavelength of 268 nm the 3 Q0 state is excited nearly exclusively. 23,48 A minor amount of the population is transferred to the 1 Q1 state via a non-adiabatic crossing, leading to a rapid cleavage of the H3 C−I bond on both PES. 49 Dissociation in the 3 Q0 state results in excited iodine fragments in the I∗ (2 P1/2 ), while the 1 Q1 state correlates to ground state I(2 P3/2 ) atoms. The branching ratio I∗ /(I∗ + I) strongly depends on the excitation wavelength and lies between 0.72–0.83 for excitation at 268 nm according to the literature. 23,24 In our scheme, single VUV probe photon ionization forming stable parent ions is possible in a broad ionization window from both states to the respective spin-orbit split ground states: 1 ˜ 1 E3/2 and 3 Q0 → X ˜ 1 E1/2 . The ionizaQ1 → X tion windows are depicted in Fig. 1 a. For bond lengths exceeding this window, the photon energy is not sufficient to ionize the molecule with a single-photon. The major I∗ (2 P1/2 ) reaction channel can be followed in this scheme up until the asymptotic region is reached, thus providing a complementary measurement to existing REMPI studies 20,25 for the reaction time of this channel. Although, the I(2 P3/2 ) channel is only visible up to a H3 C−I bond length of 2.9 Å, corresponding to a lower limit of an ionization window duration of >22 fs, retrieved from a classical trajectory calculation, it is included in our analysis. For these means, Newton’s classical equations of motion are solved for the onedimensional representation of the 1 Q1 state (ap-


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The Journal of Physical Chemistry 6a)

6b)

C 2A1

ionization window

I+ 63P2) + CH3

A

~ 2E X 1/2 ~ 2E X 3/2

C 2A1

ionization window

B 2E

I+ 61D0) + CH3

I+ 63P2) + CH3

A ~2 X E3/2

~ 2E X 1/2

3

Q0

UV pump

~ X 1A1

A 2A1

6p 62E3/2) ν1 I 62P3/2) + CH3*

2 1A1 3 A1 Q1

I 62P3/2) + CH3+

UV probe

VUV probe

1

B 2E

I+ 61D0) + CH3

A 2A1

I 62P3/2) + CH3+

[CH3+ I-]

I 62P1/2) + CH3

VUV pump I 62P3/2) + CH3

I 62P3/2) + CH3

~ 1A X 1

Figure 1: Excitation schemes and relevant electronic states for the (V)UV photodissociation of CH3 I. (a) A band dissociation: UV-pump–VUV-probe scheme. (b) D band dissociation: VUVpump–UV-probe scheme. Potential energy curves adopted from Ref. 44 (CH3 I) and Ref. 45,46 (CH3 I+ ). (V)UV absorption cross section data (logarithmic scale, black) adopted from Ref. 37, and photoelectron spectra (red) from Ref. 47.

CH3 I∗

I

probe

+

probe

CH+ 3

CH+ 3

pump

CH3 I∗

CH3 I†

I+ probe

result of the following processes. (1) Probe photon ionization of uncharged fragments, created via the initial dissociation in the A band:

probe

(2)

CH3 + I

(1) (3) Dissociative few-photon ionization, where the pump pulse resonantly excites unbound ionic states while the C-I bond is still intact:

CH3 + I

(2) Absorption of an additional pump photon, transferring population to another excited state (CH3 I† ), where subsequent dynamics are taking place. The product fragments of the latter process are then detected by probe photon ionization.

CH+ + I 3 CH3 I+ probe

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CH3 + I+

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Signal / arb. units

CH3I+

0.5 0.4

lay time. Both components are included in a data evaluation model consisting of a fast and slow decay component, with respective decay constants of 28.0 ± 2.0 fs for the dissociative ionization scheme and 478 ± 30 fs for the two UV-photon absorption-induced dynamics. The dissociative ionization yield depends strongly on the few-photon excitation cross section of the dissociative CH3 I+ ion state, which evolves in time with the geometrical distortion of the molecular bond along the reaction coordinate. The excitation from either the 1 Q1 and 3 Q0 states to the molecular ion excited A state is resonant as long as the wave packet propagates within the Franck-Condon region. The potential energy difference between the Q states and the dissociative ionic states converging to the I+ (1 D0 ) + CH3 asymptote is increasing for elongated H3 C−I bond lengths. Thus, the fast population decay component in our signal is attributed to a rapidly decreasing dissociative ionization cross section. The slower component is exceeding the molecular dissociation reaction time by more than 300 fs and no considerable offset between the fast and slow component is observed. This finding suggests that absorption of a second UV photon, while the pump pulse is still interacting with the molecule, transfers a fraction of the population to long-lived Rydberg states. This yields I+ by single VUV-photon dissociative ionization upon interrogation, similar to the excitation scheme of Durá et al. 18 , where this population transfer is observed in the CH3+ transient after IR-photon ionization in the probe step. This slowly decaying component is not observed in the delay-dependent CH3+ yield, which can be fitted by a single exponential decay component with a time constant of 34.4 ± 1.9 fs. The time scale is similar to the fast decay of the I+ ion yield, where the CH3+ A state acts as an intermediate resonance in dissociative ionization. Internal conversion from the A state to the ionic ground state is a wellknown dissociation pathway, where dynamics are taking place on a picosecond to nanosecond time scale, 45,51 but do not involve absorption of another probe photon. Since this is a single-photon probe pathway, it is expected to

τ>3Q0) = 98.2 ± 1.9 fs

CH3I+

τ>1Q1) = 28.5 ± 2.8 fs

0.3 0.2 0.1

τ>6p 2E3/2) = 80 ± 6 fs

0.0 -500 -400 -300 -200 -100

0

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Signal / arb. units

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0.3 0.2 0.1

I+

τ1 = 28.0 ± 2.0 fs τ2 = 478 ± 30 fs

τ = 124.4 ± 3.1 fs

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0.2

CH3+ 0.1

τ1 = 117.1 ± 4.8 fs

τ = 34.4 ± 1.9 fs

τ2 > 10 ps 0.0 -500 -400 -300 -200 -100

0

100 200 300 400 500

Delay / fs

Figure 2: Ion distribution of CH3 I+ and pumpprobe data sets for the CH3 I+ parent ion and its photodissociation fragments. (a) CH3 I+ distribution in the common UV+VUV focus. Positive delay: UV pulse early and VUV late. Negative delay: vice versa. (b) CH3 I+ parent ion yield. (c) I+ fragment yield. (d) CH3+ fragment yield. The first two processes are accompanied by a shift of the maximum fragment yield towards increased delays, resulting from the molecular dissociation time and have been observed in time-resolved experiments using REMPI detection schemes 16,25,50 or non-resonant few-photon IR ionization. 18 Here, the delay-dependent I+ yield (Fig. 2 c) reaches its maximum near the delay-time origin. Thus, the observed delay-dependence is interpreted considering a dissociative ionization scheme (3), dominant for small delay-times, and the excitation of high-lying Rydberg states by two UV-photon absorption in the pump step (2), which may be ionized at a later point in time, corresponding to signal at increased de-


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be the dominant contribution and is only accessible near the Franck-Condon region, as described above. Fast decay components, attributed to the dissociative ionization near the Franck-Condon region, are observed for both fragment ions with time constants less than 35 fs. This shows, that the wave packet created by the UV pulse in the A absorption band of CH3 I, leaves the Franck-Condon region in this time frame. Our scheme relies on a weak few-photon probe to avoid strong fields influencing the intrinsic reaction dynamics near the delay-time origin and provides sufficient temporal resolution to investigate these early stage reaction dynamics.

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excited with the 161-nm pump pulse, is predissociated by crossing the 2 1 A1 state. 44 The minimum of the potential near 3.7 Å is induced by the interaction between the valence state and the [CH3+ I – ] ion-pair state. Depending on the fate of the prepared vibrationally-hot wave packet, different probe schemes are relevant to the interpretation of the experiment, as presented in Fig. 1 b. When the coupling between the 6p (2 E3/2 ) state and dissociative states is neglected, a deexcitation is only possible by radiative processes evolving in the ps–ns timescale. Rydberg state population can be transferred to the first excited state of CH3 I+ by single UV-photon ionization. Such a process is observed as non-decaying contribution to the CH3+ signal, within the delay-time detection range of the single-shot technique. When a coupling between the 6p (2 E3/2 ) state and the 2 1 A1 state, as indicated in Fig. 1 b, is considered, dissociation dynamics can proceed on the 10–100 femtosecond time scale. These may be interrogated by single UV-photon absorption to form stable molecular ions, in the molecular ionization window. Additionally, dissociative two-photon ionization may result in a delay-dependent CH3+ and I+ fragment yield. The CH3 I+ yield shown in Fig. 2 b indicates a delay dependence in the negative time-delay range, where the VUV pulse arrives early (left side). The exponential decay component with a time constant of 80 ± 6 fs does not contribute strongly to the overall signal, indicating a lower excitation probability compared to the Q state excitation, when the spectrum of the 161-nm pulse is considered. Figure 1 b highlights the single UV-photon ionization window for a dissociation in the 2 1 A1 state, which is attributed to the observed delay-dependent parent ion signal. The 3 A1 state is indicated for comparison, as it is a known dissociation pathway for the lower Rydberg states. 44 In this case dissociation through this pathway, would lead to a much shorter observation window for the parent compared to the 2 1 A1 state, due to the rapidly increasing ionization energy. The conclusion, that the 2 1 A1 state is responsible for predissociation of the 6p (2 E3/2 ) Rydberg state, is substantiated by the delay-

Dissociation after VUV Rydberg Excitation While the UV-induced dissociation dynamics are initiated by direct excitation of the dissociative Q states, reaction dynamics after VUV photon absorption are enabled by the presence of closely spaced Rydberg states. 5 While previous studies 5,9,10 have focused on dynamics initiated via two-UV photon absorption in these highly excited molecular states, the excitation scheme used in this study is tailored to access the ν1 vibrationally-excited 6p (2 E3/2 ) Rydberg state by single VUV-photon excitation at 161 nm. 37 The predissociative nature of other Rydberg states, such as the 6s (2 E1/2 ), 6p (2 E1/2 ) and 7s (2 E3/2 ) has been studied previously both experimentally, as well as theoretically. 5,44 These studies provide evidence for sub-picosecond dynamics, leading to a decomposition of the molecule. The analysis of reaction dynamics after Rydberg excitations below 170 nm relies mostly on spectroscopic information. 37,47 To the best of our knowledge, no theoretical studies about the potential energy surfaces relevant to the highly excited molecular states accessed by the 161-nm pump pulse have been published at the time this experimental study has been conducted. The absence of the relevant theoretical data complicates a thorough analysis of the excited wave packet dynamics. However, there are indications, that the 6p (2 E3/2 ) Rydberg state,


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dependent signals observed for the I+ and CH3+ fragments. Both yields exhibit exponential decays with a time constant of τ (I+ ) = 124.4 ± 3.1 fs and τ (CH+ 3 ) = 117.1±4.8 fs. The origin of these time constants is ascribed to dissociative ionization initiated by absorption of two probe photons. Relevant ionic states, as well as a window indicated by the blue double arrows, are shown in Fig. 1 b. Similar as in our study 40 of the VUV-induced dissociation dynamics of O2 this pathway extends the parent species’ ionization window, which is in accordance with dissociation through the 2 1 A1 state. An additional long-lived contribution to the CH3+ yield, does not decay in the delay-time window of ±500 fs accessible by the singleshot technique. Similar to the UV-pump VUVprobe scheme, a single-photon ionization from the 6p (2 E3/2 ) Rydberg state to the first excited ion state may create CH3+ fragments by internal conversion. The long decay constant of this contribution is an indication, that a fraction of the initial Rydberg state population is either transferred to other not predissociated Rydberg states or to the vibrational ground state of the 6p (2 E3/2 ) state and can be interrogated at longer delays. The molecular dynamics evolving in 80–120 fs is attributed to a partial population transfer from the Rydberg state to the dissociating 2 1 A1 state, where the major part of the population is transferred.

population can be excited in the complete delay time window accessible. This time-resolved study provides a step to further understand dissociation dynamics enabled by highly excited Rydberg states relevant in environmental chemistry. Studying VUV dissociation dynamics in the CH3 I model system is an important milestone before applying the presented new methodology to more complex organic and biological systems and getting closer to unravel non-adiabatic dynamics proceeding through sequences of conical intersections. The relevance of the 2 1 A1 state of CH3 I, which was not observed experimentally up until the date of preparation of the manuscript due to unfavorable Franck-Condon factors, 44 in the dissociation dynamics was shown. Its distinct shape, caused by the interaction between Rydberg, valence and ion-pair states, is yet another aspect, where iodomethane may serve as an important prototypical system in experiments and theoretical computations alike. In the future, this information can be extended by combining the advantages of the presented single-shot technique, which is able to discern sub-10-fs dynamics with high statistical fidelity, with specifically tailored interrogation schemes similar to the REMPI technique. An important next step to achieve this goal is the design and implementation of wavelength tunable few-fs table-top radiation sources in the vacuum ultraviolet spectral range, which allow elucidating few-fs dissociation dynamics and may be combined with time-resolved photoelectron spectroscopy and coincidence techniques. These schemes will enable to select the initially excited state and to precisely adjust the interrogation scheme to the experiment, completely avoiding perturbations of the system under study, potentially imposed by the high intensities present in multiphoton ionization techniques.

Outlook & Conclusion In summary, single-shot UV/VUV pump probe spectroscopy was utilized to reexamine the A band dissociation dynamics of iodomethane with a sub 10-fs temporal resolution and simultaneously study the dynamics of the highly excited 6p (2 E3/2 ) Rydberg state. By avoiding strong multi-photon ionization probe fields, sub-30 fs wave-packet dissociation dynamics within the Franck-Condon region were discerned with the 268-nm pump pulse. Additionally, with a 161-nm pump pulse a partial population transfer from the initially excited Rydberg state was observed, which leads to dissociation via the 2 1 A1 state, while part of the

Acknowledgement We gratefully thank the German Science Foundation (DFG) within the the GRK 1355 (Physics with new coherent light sources) and the SFB 925 (Light induced dynamics and control of correlated quantum systems) subproject A2, as well as the Hamburg


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Centre for Ultrafast Imaging (CUI) for financial support. Also we are grateful to Aleksey Alekseyev for his valuable input regarding possible excitation schemes and fruitful discussions.

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(10) Gedanken, A.; Robin, M. B.; Yafet, Y. The methyl iodide multiphoton ionization spectrum with intermediate resonance in the A-band region. J. Chem. Phys. 1982, 76, 4798–4808. (11) Khundkar, L. R.; Zewail, A. H. Picosecond MPI mass spectrometry of CH3 I in the process of dissociation. Chem. Phys. Lett. 1987, 142, 426–432.

Supporting Information Available

(12) Chandler, D. W.; W. Thoman Jr., J.; Janssen, M. H. M.; Parker, D. H. Photofragment imaging: The 266 nm photodissociation of CH3 I. Chem. Phys. Lett. 1989, 156, 151–158.

Further information about the determination of the instrument response function for the experiment is presented in the Supporting Information.

(13) Dantus, M.; Janssen, M. H. M.; Zewail, A. H. Femtosecond probing of molecular-dynamics by massspectrometry in a molecular-beam. Chem. Phys. Lett. 1991, 181, 281–287.

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Graphical TOC Entry Valence excitation

Δτ Rydberg excitation

CH3I+

268 nm

161 nm


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Time-Resolved Dissociation Dynamics of ... - ACS Publications

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