Delayed Relaxation of Highly Excited Cationic States in Naphthalene

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A: Kinetics, Dynamics, Photochemistry, and Excited States

Delayed Relaxation of Highly Excited Cationic States in Naphthalene Geert Reitsma, Johan Hummert, Judith Dura, Vincent Loriot, Marc J.J. Vrakking, Franck Lepine, and Oleg Kornilov J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b10444 • Publication Date (Web): 19 Mar 2019 Downloaded from http://pubs.acs.org on March 28, 2019

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The Journal of Physical Chemistry

Delayed Relaxation of Highly Excited Cationic States in Naphthalene †

Geert Reitsma,

†

Johan Hummert,

†

Vrakking,

†Max-Born-Institut

†

Judith Dura,

Franck Lépine,

‡

Vincent Loriot,

and Oleg Kornilov

‡

Marc J. J.

∗,†

für Nichtlineare Optik und Kurzzeitspektroskopie, Max-Born-Strasse 2A, D-12489 Berlin, Germany.

‡Institut

Lumière Matière, Université Lyon 1, CNRS, UMR 5306, 10 rue Ada Byron, 69622 Villeurbanne Cedex, France.

E-mail: [email protected]

1

ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Abstract Rapid energy transfer from electronic to nuclear degrees of freedom underlies many biological processes and astrophysical observations. The eciency of this energy transfer depends strongly on the complex interplay between electronic and nuclear motion. In this study we report two-color pump-probe experiments that probe the relaxation dynamics of highly excited cationic states of naphthalene, a prototypical polycyclic aromatic hydrocarbon molecule, which are produced using wavelength-selected, ultrashort extreme ultraviolet pulses. Surprisingly, the relaxation lifetimes increase with the cationic excitation energy. We postulate that the observed eect is the result of a population trapping that leads to delayed relaxation.

Introduction Ionization by extreme ultraviolet (XUV) and X-ray photons often involves excitations to very high-lying electronic states of the absorbing molecules. The subsequent relaxation processes have importance both in biological environments (because of the generation of free electrons) and in astrochemistry (because of the role of XUV-induced fragmentation).

Direct time-

domain observation of electronic and structural dynamics in highly electronically excited molecular systems has recently become experimentally accessible

14

due to major advances

in the generation of the ultrashort XUV/X-ray laser pulse at free electron lasers or via high-harmonic generation (HHG).

5

A class of larger molecules, which has recently come into the focus of this research are Polycyclic Aromatic Hydrocarbons (PAHs).

6

These molecules exhibit a range of excited

states in their XUV ionization spectrum as a result of electron correlation.

7

Recently, we have

reported a time-resolved mass-spectrometric study of several PAHs showing the existence of an ultrafast decay pathway for the highly excited cationic states.

6

An increase of the

relaxation timescale with increasing size of the PAHs was observed. Follow-up experiments used time-resolved photoelectron spectroscopy and showed the increase of lifetimes with

2


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cationic state energy.

8

This emphasized the role of the high density of cationic states and

was supported by many-body quantum calculations. Here we extend the previous work by employing an XUV time-delay-compensating monochromator (TDCM) in order to prepare molecular cations using a well-dened XUV photon energy. We investigate the relaxation dynamics of highly excited cationic states of one of the PAHs - naphthalene - as a function of the XUV photon energy.

Naphthalene has an

ionization potential of 8.1 eV and a double ionization potential of 21.5 eV.

9

Upon ioniza-

tion using a suciently high photon energy, naphthalene cations dissociate.

C2 H2 -loss 19 eV.

H, H2 ,

and

channels appear around 15 eV and even more extensive fragmentation sets in at

1015

Naphthalene's electronic structure has been studied both experimentally,

using photoelectron spectroscopy, and theoretically.

16,17

by

7,1821

Figure 1: Ionization spectrum of naphthalene reproduced from the data of Ref.

7

The pole

strength corresponds to the contribution of 1-hole congurations to the wavefunction of electronic states that have a binding energy that is plotted along the horizontal axis. The relevant bands are labeled with the orbital that is ionized to populate the main state and its satellites. The black dashed line indicates the double ionization threshold of 21.5 eV.

9

The colored arrows are drawn according to the photon energies used in the experiment and allow an assessment of the states that can be reached at each photon energy. The same color coding for the photon energies is used throughout the paper.

7

In Figure 1 (reproduced from the data of Ref ) the ionization spectrum of naphthalene is shown as a stick diagram.

The height of the sticks represents the contribution of 1-

3



Page 4 of 26

hole ionization leading to a given electronic state of the naphthalene cation.

Below an

ionization energy of about 15 eV, all cationic states are well described by a dominant 1hole electronic conguration. However, for higher ionization energies a broad distribution of satellite states appears in the ionization spectrum, which are described by multiple electronic congurations and correspond to a situation, where removal of one electron from the molecule is accompanied by excitation (i.e. shake-up) of other electrons. In the present work we use photon energies in the range of 20-30 eV, where shake-up states dominate the ionization spectrum.

We studied the lifetimes of the excited states that were formed in the XUV

ionization process by probing the cation with a time-delayed IR laser pulse and measuring fragment ion and dication yields as a function of two-color XUV-IR delay. Our results conrm the ultrafast relaxation of highly excited cationic states of naphthalene

6

and demonstrate a

trend of increasing lifetimes with increasing XUV photon energy. This observation, together with those of Ref.

6

allows us to postulate, that PAHs belong to a class of molecules where

population can remain trapped in excited states. Some of the species in excited states relax promptly, while the relaxation of others is delayed.

As elaborated in the discussion, this

conjecture could explain the increase of the observed relaxation timescales in aforementioned experiments.

Experimental The TDCM beamline has been described in detail elsewhere.

4

In short, the output of a Ti:Sa

laser system (central wavelength 795 nm, pulse duration 25 fs, repetition rate 1 kHz) was split into two parts. The rst part, with a pulse energy of 1.5 mJ, was used to generate XUV laser pulses by means of HHG. This XUV beam was monochromatized and recompressed to a pulse duration of

≤ 25 fs by the TDCM before entering a velocity map imaging spectrometer

(VMIS). To access excited states of the naphthalene cation close to the double ionization threshold (21.5 eV), harmonics 13 to 19 were used with photon energies

4


Eph =

20.4, 23.6,

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26.7, and 29.8 eV. The spectral bandwidth transmitted by the TDCM was approximately 0.3 eV. The second part of the Ti:Sa laser output passed a delay stage and was routed to the VMIS through the vacuum system of the TDCM. Just before the VMIS, the IR beam was recombined with the XUV beam by means of an annular mirror. The IR intensity used in the experiment was set to 2-4

TW/cm2

in order to avoid extensive ionization by the probe

pulse only. Since high target densities are required to achieve a good signal-to-noise ratio, the VMIS was equipped with a repeller with an integrated oven (operated at 330 K), ensuring sucient target density and a well-dened interaction region. In the main set of experiments presented in this article the VMIS was operated in an ion time-of-ight (TOF) detection mode.

22

For

measurements of the XUV-IR cross correlation and a calibration of the XUV spectra, the electron imaging mode of the VMIS was used. The XUV-IR cross correlation was measured detecting sidebands in argon photoelectron spectra, which were also used to optimize the temporal and spatial overlap between the XUV and IR pulses.

The measured XUV-IR

cross-correlation was around 40 fs for harmonics 13, 15, and 17 and 30 fs for harmonic 19. Experimental mass spectra were acquired for each of the four XUV photon energies as a function of XUV-IR pump-probe delay.

The delay was scanned between -100 and 400

fs (positive delays correspond to the IR pulse arriving after the XUV pulse).

Each mass

spectrum was averaged over 15000 laser shots (15 s) and every time delay scan was repeated in both forward and backward directions multiple times (i.e. 4 to 9 times).

Results Figure 2 displays mass spectra of naphthalene resulting from two-color XUV-IR ionization (using 20.4 eV XUV photons, i.e. harmonic 13) in the region of mass-to-charge ratios between

m/q =26

and

m/q =80.

Please, note that in the preparation of Figure 2 one-color

contributions induced by the XUV and the IR pulses have been subtracted. The yields of

5



Figure 2: XUV-IR delay-dependent mass spectrum resulting from ionization of naphthalene for

Eph =

20.4 eV and

EIR =

1.6 eV. A positive XUV-IR delay means that the

IR pulse comes after the XUV pulse. The gure shows projections of the mass spectrum, that is collected for an XUV-IR delay of 0 fs, and of the time-dependent yield of the 2+ naphthalene dication C10 H8 .

6


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many ionic products strongly increase when both pulses are overlapped in time and show an exponential decrease when the IR pulse is delayed with respect to the XUV pulse. The dication yield (m/q =64) is plotted versus delay time in the projected plane on the left (blue line). The fragments

+ + + + + + + C3 H+ 3 , C4 H2 , C4 H3 , C4 H4 , C5 H3 , C5 H5 , C6 H2 , C6 H3 ,

and

C6 H+ 4

all

show similar dynamics. XUV-IR delay-dependent mass spectra as shown in Figure 2 were recorded for all four chosen XUV photon energies. (m/q

Figure 3 displays the two-color time-dependent dication

= 64) yields extracted from these measurements (in each case normalized to the signal

maximum). We note that the XUV-only contribution (already subtracted in Figure 3) is zero for

Eph =

20.4 eV, but increases strongly with increasing photon energy. This is in line with

the sharp increase of the naphthalene dication yield above the double ionization threshold of 21.5 eV reported by Eland and co-workers.

23

At the same time, the photoionization cross

section of naphthalene decreases beyond 27 eV.

24

These two eects conspire to reduce the

signal-to-noise ratio for higher XUV photon energies, which is in particular visible in the data for

Eph =

29.8 eV.

To extract a time constant for the decay of the ion yields with the XUV-IR delay we use a t function that consists of the sum of an exponential decay and a non-decaying step function, which represents pump-probe signals with lifetimes much longer than the range of delays investigated in the present experiments. This t model is convoluted with a Gaussian that represents the XUV-IR cross correlation, i.e.:

2

F (t) = e

where

t − 2σ 2

∗ H(t − t0 ) Ae

H(t) is the Heaviside step function, A and τ

for the exponential decay,

B

−(t−t0 ) τ

+B

are the amplitude and the time constant

is the amplitude of the step function,

between the XUV and IR pulses and

√ 2 2 ln 2σ

(1)

t0

is time of zero delay

is the full-width-at-half-maximum (FWHM)

of the XUV-IR cross correlation. We apply a multidimensional least squares tting procedure

7



Dication yield

1.5

1.5

Eph = 20.4 eV Cross correlation

1.0


1.0

0.5

0.5

0.0

0

1.5 Dication yield

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 26

200

400

0

1.5


1.0

0.0

200

400


1.0 0.5

0.5

0.0 0.0

0

200 Delay (fs)

400

0.5

0

200 Delay (fs)

400

Figure 3: The four time-dependent naphthalene dication yields as a function of XUV-IR delay for dierent XUV photon energies used in the experiment, corresponding to the use of harmonics 13-19.

in which the nine aforementioned fragments and the dication are taken into account. and

B

σ

are used as global t parameters (varied simultaneously for all fragments) and

are used as local t parameters (allowed to dier for dierent fragments).

τ , t0 , A

and

Taking the

time constant for the decay as a global parameter is based on the observation that the same dynamics appears to be present in the time-dependent ion yields of all these ten features in the mass spectrum. It was observed that the time constants do not change appreciably (within the error bars) when allowing individual time constants for each feature. The tted relaxation timescales resulting from this procedure are plotted in Figure 4 for all the individual scans that were performed. and represents a

1×σ

deviation. Figure 4 permits a rather accurate determination of the

relaxation timescale for with

Eph =

20.4 eV is

The error bars are derived from the t

Eph =

27 ± 2

20.4 and 23.6 eV. The relaxation timescale upon ionization

fs while the relaxation timescale upon ionization with

23.6 eV is 10 fs slower, namely

37 ± 3

fs. The accuracy for

Eph =

Eph =

26.7 and 29.8 eV is lower,

but the relaxation times that are obtained for dierent scans are consistent within the error

8


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Figure 4: Relaxation timescales obtained using the scan-by-scan multidimensional tting procedure described in the text. The error bars are derived from the t error. The presented value is the weighted average of the individually determined relaxation timescales and its error bar is the standard deviation of the weighted average. Remarkably, the measured relaxation timescales tend to increase with the XUV photon energy used.

bars, yielding

37 ± 2

fs and

43 ± 5

fs for

Eph =

26.7 and 29.8 eV, respectively.

Another look at the data is provided by Figure 5, where fragment mass spectra at the time delay corresponding to the maximum of the signal (≈

20 fs)

are plotted for the four

measurements with dierent photon energies. As before, XUV-only contributions are subtracted. patterns.

Changing the ionization photon energy substantially changes the fragmentation For

Eph =

20.4 eV extensive fragmentation distributes the measured ion signal

with the signal distributed over many channels. For higher photon energies, however, two mass peaks dominate the spectra: cation and

m/q = 51,

m/q = 64,

which corresponds to

which corresponds to the unfragmented di-

C2 H2 -loss

from the dication.

12

On the basis

of these measurements, we conclude that the cationic states that are responsible for the observed fragmentation induced by higher XUV photon energies are substantially dierent from the cationic states that are responsible for the fragmentation at lower XUV photon energies.

9



Page 10 of 26

Figure 5: Two-color XUV-IR time-of-ight mass spectra at the time corresponding to the maximum of the two-color signal (XUV-IR delay of

≈ 20 fs).

At each photon energy, the

spectra are normalized to the intensity of the dication signal at (m/q = 64). The change in the fragment ion yields relative to the dication yield shows that the cationic states that are mainly responsible for the observed ions at high XUV photon energy dier from the cationic states that dominate the response at low XUV photon energy.

The photon order of the IR probing process can be estimated by recording mass spectra as a function of IR intensity.

In Figure 6 intensity-dependent dication yields recorded at

an XUV-IR delay of 20 fs are displayed on a double-log scale for all four XUV photon energies. The data are well-tted by a linear slope and suggest that the probe step requires the absorption of two IR photons for all XUV photon energies used in the experiments.

Discussion The interpretation of the experimental data is facilitated by a comparison with the theoretical photoionization spectra (Figure 1). XUV photons

Eph =

20.4 eV can access the electronic

band of the states centered at 19.8 eV and corresponding to the removal of an electron from the orbitals

6ag , 5b1u

and

4b2u .

In time-resolved photoelectron experiments of Ref.,

8

performed with broadband XUV pulses, three timescales were observed and assigned to relaxation of these three cationic states. However, the relaxation timescales observed in the current experiment cannot be attributed only to these excited cationic states.

This is a

necessary consequence, in particular, of the experimental data displayed in Figure 5, which show a strong variation of the two-color fragmentation with photon energy. The dominance

10


Page 11 of 26

Dication Yield

103

Eph = 20.4 Slope = 2.0±0.5

Eph = 26.7 Slope = 1.6±0.5 103

101

10

1

101

1013

103 Dication yield

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1013

Eph = 23.6 Slope = 2.3±0.5

Eph = 29.8 Slope = 1.7±0.5 103

101

10

1

101

1013 Intensity (W/cm2)


Figure 6: Two-color XUV-IR dication yields as a function of the IR intensity in the experiment, measured at an XUV-IR timed delay of 20 fs. The IR intensity was estimated from a measurement of a pulse energy behind the endstation. Each data point represents an averaging over 30000 laser shots.

of the dication signal and the signal corresponding to

Eph =

with the photon energy between between

Eph =

26.7 eV and

Eph =

20.4 eV and

C2 H2 -loss Eph =

from the dication increases

26.7 eV and remains constant

29.8 eV. This can only be explained if, upon removal of

the electron by the XUV pulse, the dominant cationic state that is left behind changes with XUV photon energy, and this leads to the conclusion, that a dominant contribution to the detected signals in experiments at higher photon energies comes from relaxation processes involving higher-lying states. These states could correspond to removal of an electron from the orbitals

3b3g , 5ag

26.7 eV), and

4ag

(possible for

Eph = 23.6 eV), 4b1u , 3b2u

(additionally possible for

(additionally possible for

Eph =

Eph = 29.8 eV). Extending this argumentation we

conclude that the increase of the measured relaxation timescales upon increasing the XUV photon energy (see Figure 3) is a consequence of the contributions of these higher-lying states. In principle, one might expect that the lifetime of excited states decreases with excita-

11



Page 12 of 26

tion energy, because the higher density of coupled electronic states increases the number of conical intersections close to the Franck-Condon region, facilitating the relaxation process. However, the trend observed in our experimental data is obviously opposite.

Let us con-

sider the present observations together with the previous observation of Marciniak et al,

6

where time-dependent dication yields were reported for a range of PAH molecules in XUV-IR pump-probe experiments with a xed, broadband XUV spectrum and increasing lifetimes for increasing molecule size were observed. The combination of both experiments allows us to make a striking observation: in both cases the measured lifetimes increase (rather than decrease) with an increasing density of excited states. In the present work the increase in density of states is the result of increasing the XUV photon energy, accessing an energy range with a higher density of satellite states (see Figure 1), whereas in Ref.

6

the increase of the

size of the molecule at xed XUV spectrum led to a higher density of states. The question arises, whether the apparent dependence of the relaxation lifetime on the density of states manifests a more general phenomenon. In Ref.

25

it was proposed that a

trapping of population occurs in radiationless decay of excited states coupled to a common continuum as a result of a separation of relaxation timescales induced by interference in the continuum:

some states relax promptly, while relaxation of others is delayed.

The

formalism of this trapping eect was further developed in a series of publications by Remacle, Levine and co-workers, and applied to phenomena like delayed ionization, dissociation

27,28

and pre-dissociation.

29

26

unimolecular

The common aspect in all these works is that the

relaxation of an excited molecule is described in terms of the quantum dynamics of a system consisting of a large number of bound states that are coupled to a small number of relaxation channels. Under the assumptions that (i) the number of relaxation channels is much smaller than the number of bound states and (ii) the coupling among the bound states is much weaker than the coupling of these bound states to the relaxation channels, the predicted relaxation timescales form a bimodal distribution including prompt and delayed (trapped) components. The larger the number of bound states, the stronger the separation between

12


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the prompt and delayed timescales.

We propose that this general mechanism may be at

play in the relaxation towards the electronic ground state of highly-excited PAH cations. In this case the bound states are represented by the highly electronically excited manifold of shake-up states that are populated upon photoionization by the XUV laser pulse. Under the assumption that these states are coupled (at conical intersections) to the ground electronic state, the latter represents a "bottleneck" for the relaxation process.

If the condition (ii)

is also fullled, implying that the excited states are more strongly coupled to the ground state than to each other, the relaxation of the excited molecule would be predicted to exhibit a multiexponential relaxation dynamics, where part of the population will decay very fast (prompt relaxation to the ground state), while another part will be trapped in the excited manifold and will decay signicantly slower. The results of Refs.

27,28

suggest that in this case the relaxation timescale of the delayed

component scales linearly with the density of initial bound states.

Note that this does

not necessarily lead to a monotonic increase of the relaxation time with photon energy, as the density of states may increase non-monotonically. time increases from

27 ± 3 fs

to

37 ± 3 fs

In the present work the relaxation

by tuning the photon energy from 20.4 to 23.6 eV,

remains constant within the error bars up to the photon energy of 26.7 eV and then grows to

43 ± 5 fs

for the photon energy of 29.8 eV. Taken in the full range of photon energies this

corresponds to the increase of the relaxation time by a factor of

1.6 ± 0.3.

If one makes

the crude assumption of a constant density of vibrational states for all excited electronic states, an increase of the relaxation timescale by a factor 1.6 is estimated from the increase in the density of electronic states predicted in Ref.,

7

which lists 14 electronic states for the

band around 19.8 eV and 22 electronic states for the broad band above the double ionization threshold. Similarly, in the work of Marciniak et al fs to

6

the relaxation time increases from

29±4

55±22 fs (ratio of 1.9±0.7) when changing from naphthalene to tetracene, in agreement

with the increase of the PAH size (a factor 1.8 in molecular weight). While this numerical agreement may be a coincidence, it speaks in favor of the our proposal, that the increase of

13



lifetimes as observed here for naphthalene and in Ref.

6

Page 14 of 26

for other PAHs, is the manifestation

of quantum mechanical trapping of population in the excited state manifold induced by the "bottleneck" ground state (we note that the follow-up results of Ref.

8

are also consistent with

this mechanism). Within this interpretation we conclude that our current experiments lack the time-resolution to resolve the fast component, but are able to track the slow component of the multiexponential relaxation dynamics.

The fast component could presumably be

observed in experiments combining attosecond XUV pump pulses and few-cycle IR probe pulses.

Conclusion We have performed XUV-IR pump-probe experiments using a femtosecond source of wavelengthselected XUV pulses investigating the ultrafast relaxation dynamics of highly excited cationic states of naphthalene. The experimental results show a trend towards increasing lifetimes for higher-lying excited states. In combination with the recent results of Ref.

6

this leads us

to propose a timescale separation in electronically excited cationic states of PAHs, similar to models previously proposed for other molecular relaxation processes, such as unimolecular dissociation

27

and pre-dissociation.

29

The assumptions of such a model, namely, a strong

coupling to the relaxation channel and a weak to moderate coupling within the excited state manifold are plausible for PAHs, making them a candidate for the observation of the timescale separation phenomenon.

Acknowledgement G.R. thanks the Netherlands Organization for Scientic Research (NWO) for nancial support (Rubicon 68- 50-1410). J. H. and O. K. acknowledge support of the Deutsche Forschungsgeminschaft (KO 4920/1-1). V. L. and F. L acknowledge support of CNRS, ANR-16-CE300012 Circé.

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Graphical TOC Entry

19


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The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38


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1.5

Page 23 of 26

Dication yield

Dication yield

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44

1.5



1.0

1.0

0.5

0.5

0.0

0.0

1.5

0

200

400


1.0


0

1.5

200

400


1.0 0.5

0.5

0.0 0.0

0

200 Delay (fs)

400

0.5


0

200 Delay (fs)

400

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44


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103

Dication yield

Dication Yield

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44


Eph = 20.4 Slope = 2.0±0.5

Page 26 of 26

Eph = 26.7 Slope = 1.6±0.5 103

101

10

1

103

101

1013 Eph = 23.6 Slope = 2.3±0.5

1013 Eph = 29.8 Slope = 1.7±0.5

103 101

10

1

1013

101

Intensity (W/cm2)



Delayed Relaxation of Highly Excited Cationic States in Naphthalene

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