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Experimental studies of decays at long timescales Patrick Rousseau patrick.rousseau@unicaen.fr EMIE-UP summer school patrick.rousseau@unicaen.fr 1 Multiscale Dynamics in Molecular Systems Introduction Instru strume ment ntatio ion

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patrick.rousseau@unicaen.fr EMIE-UP summer school “Multiscale Dynamics in Molecular Systems” 1

Experimental studies

  • f decays at long timescales

Patrick Rousseau

patrick.rousseau@unicaen.fr

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patrick.rousseau@unicaen.fr EMIE-UP summer school “Multiscale Dynamics in Molecular Systems” 2

Introduction Instru strume ment ntatio ion Me Metastable d disso sociation

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Radi diati tive c coo

  • oling

Vibrati tiona nal e electr ctron d detachm chment Slow iso isome merisa sati tion

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patrick.rousseau@unicaen.fr EMIE-UP summer school “Multiscale Dynamics in Molecular Systems” 3

Complex systems

They posses a large number of degrees of freedom: →! numerous relaxation channels are expected

  • in competition
  • at difgerent timescales
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patrick.rousseau@unicaen.fr EMIE-UP summer school “Multiscale Dynamics in Molecular Systems” 4

Jablonski diagram

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patrick.rousseau@unicaen.fr EMIE-UP summer school “Multiscale Dynamics in Molecular Systems” 5

Complex systems

They posses a large number of degree of freedom: →! numerous relaxation channels are expected

  • in competition
  • at difgerent timescales

Today we will focus on really long timescales (higher than ns).

10-15 s 10-12 s 10-9 s 10-6 s 10-3 s 1 s 103 s

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patrick.rousseau@unicaen.fr EMIE-UP summer school “Multiscale Dynamics in Molecular Systems” 6

Long timescale

At such long timescale, the system can be described by statistical physics. From the experimental point of view, we need to keep the control on the system BUT in 1 µs, a thermal water molecule runs through about 1 mm. One need to store the molecular system in order to study longer timescales.

10-15 s 10-12 s 10-9 s 10-6 s 10-3 s 1 s 103 s

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patrick.rousseau@unicaen.fr EMIE-UP summer school “Multiscale Dynamics in Molecular Systems” 7

Intr trodu ducti ction Instrumentation Me Metastable d disso sociation

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Radi diati tive c coo

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Vibrati tiona nal e electr ctron d detachm chment Slow iso isome merisa sati tion

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patrick.rousseau@unicaen.fr EMIE-UP summer school “Multiscale Dynamics in Molecular Systems” 8

Time-of-fmight spectrometer

Indeed for relatively “short” timescales (µs range), one may study the so- called delayed fragmentation using time-of-fmight mass spectrometer. With refmectron confjguration, one can observe decays occuring in the fjrst fjeld-free region.

ToF

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patrick.rousseau@unicaen.fr EMIE-UP summer school “Multiscale Dynamics in Molecular Systems” 9

Storage devices

For longer timescales, the fjeld-free region length required is too high. →! one needs to consider alternative confjgurations (multiple refmections, ring…) Storage rings based on magnetic fjeld existed since several decades. However, as the molecular system can be heavy, it is important to consider instead electrostatic device with no mass limitation. In 1997, two families of electrostatic storages devices emerge:

  • electrostatic ion beam trap, D. Zajfman et al. in Israel
  • electrostatic storage ring, L. H. Anderson et al. in Danmark
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patrick.rousseau@unicaen.fr EMIE-UP summer school “Multiscale Dynamics in Molecular Systems” 10

Electrostatic ion beam trap

The idea is to use two electrostatic mirrors in order to obtain multiple refmection of the beam.

  • compact design (~ 1m)
  • long storage time in UHV condition (100 ms to s)
  • D. Zajfman et al.
  • Phys. Rev. A 55 (1997) R1577
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patrick.rousseau@unicaen.fr EMIE-UP summer school “Multiscale Dynamics in Molecular Systems” 11

Electrostatic ion beam trap (2)

  • D. Zajfman et al.
  • Phys. Rev. A 55 (1997) R1577

t t Pick-up signal Neutral detector

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patrick.rousseau@unicaen.fr EMIE-UP summer school “Multiscale Dynamics in Molecular Systems” 12

Electrostatic storage ring

The use of electrostatic steering elements removes the mass limitation.

  • S. P. Moller
  • Nucl. Instrum. Methods A 394 (1997) 281
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patrick.rousseau@unicaen.fr EMIE-UP summer school “Multiscale Dynamics in Molecular Systems” 13

Electrostatic storage ring (2)

  • S. P. Moller
  • Nucl. Instrum. Methods A 394 (1997) 281

t

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patrick.rousseau@unicaen.fr EMIE-UP summer school “Multiscale Dynamics in Molecular Systems” 14

Intr trodu ducti ction Instru strume ment ntatio ion Metastable dissociation Radi diati tive c coo

  • oling

Vibrati tiona nal e electr ctron d detachm chment Slow iso isome merisa sati tion

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patrick.rousseau@unicaen.fr EMIE-UP summer school “Multiscale Dynamics in Molecular Systems” 15

Metastable fragmentation

Multiply charged fullerene can accomodate the charge excess. →! they are metastable on the µs timescale. This is due to the presence of a fjssion barrier associated with a transition state during the dissociation. Theoretical predictions stated that is stable while the dissociation energy is favourable from .

[S. Díaz-Tendero et al., Phys. Rev. Lett. 95 (2005) 013401]

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patrick.rousseau@unicaen.fr EMIE-UP summer school “Multiscale Dynamics in Molecular Systems” 16

Metastable decay of

  • H. da Silva et al.
  • Phys. Rev. A 90 (2014) 032701

Using a long extraction region (several cm), it is possible to observe decays

  • n the µs range.

ToF

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patrick.rousseau@unicaen.fr EMIE-UP summer school “Multiscale Dynamics in Molecular Systems” 17

Metastable decay of : theory

  • H. da Silva et al.
  • Phys. Rev. A 90 (2014) 032701

Based on the Weisskopf theory, a fragmentation model is obtained. The dissociation rate depends on the dissociation energy and fjssion barrier energy. Competition between dissociation by emission of neutral/charged C2. For higher charged states (q ≥ 4), one can expect a delayed fjssion.

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patrick.rousseau@unicaen.fr EMIE-UP summer school “Multiscale Dynamics in Molecular Systems” 18

Metastable fjssion of

It is possible to deduce the lifetime of the metastable states studying their delayed fragmentation.

  • H. Lebius et al.
  • Phys. Scr. T80 (1999) 197
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patrick.rousseau@unicaen.fr EMIE-UP summer school “Multiscale Dynamics in Molecular Systems” 19

Power law decay

Complex molecules with a broad energy distribution →! population of many initial excited states →! many difgerent exponential decays The emission rate is given by Considering that the energy distribution decays exponentially from an initial one broad enough to be considered as constant If is strongly peaked at its maximum It may be necessary to include a second term to the exponent

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patrick.rousseau@unicaen.fr EMIE-UP summer school “Multiscale Dynamics in Molecular Systems” 20

Metastable decay of : higher E

  • K. Hansen and E. E. B. Campbell
  • J. Phys. Chem. 104 (1996) 5012

Delaying the extraction of the ion into the ToF, one can study longer timescale. Evaporation model fails to fjt data →! competitive process

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patrick.rousseau@unicaen.fr EMIE-UP summer school “Multiscale Dynamics in Molecular Systems” 21

Take home message #1

Coupling between electronic and vibrational degrees of freedom by internal conversion. →! hot species are produced Decay through:

  • Dissociation
  • thermoionic electron emission

Evaporative model may be applied power law decay if broad initial energy distributions However some discrepancies may appear →! competitive process

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patrick.rousseau@unicaen.fr EMIE-UP summer school “Multiscale Dynamics in Molecular Systems” 22

Intr trodu ducti ction Instru strume ment ntatio ion Me Metastable d disso sociation

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Radiative cooling Vibrati tiona nal e electr ctron d detachm chment Slow iso isome merisa sati tion

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patrick.rousseau@unicaen.fr EMIE-UP summer school “Multiscale Dynamics in Molecular Systems” 23

Radiative cooling

Beside the dissociation, the internal energy can be lowered via electronic, vibrational and/or rotational transitions →! emission of one photon →! radiative cooling

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patrick.rousseau@unicaen.fr EMIE-UP summer school “Multiscale Dynamics in Molecular Systems” 24

Radiative cooling of fullerene

The radiative cooling quenches the thermoionic electron emission.

  • J. U. Andersen et al.
  • Eur. Phys. J. D 17 (2001) 189

Neutral counts Time [s]

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patrick.rousseau@unicaen.fr EMIE-UP summer school “Multiscale Dynamics in Molecular Systems” 25

Poincaré fmuorescence

The radiative cooling via vibrational transitions is associated with IR photons and long timescale (ms). It was proposed that inverse internal conversion (IIC) can lead to a fast radiative cooling by recurrent fmuorescence.

  • A. Léger, P. Boissel, L. d’Hendecourt
  • Phys. Rev. Lett. 60 (1988) 921
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patrick.rousseau@unicaen.fr EMIE-UP summer school “Multiscale Dynamics in Molecular Systems” 26

Fast radiative cooling of anthracene

Using a compact storage ring, the MINI-RING in Lyon, short revolution times (few µs) are accessible.

  • S. Martin et al.
  • Phys. Rev. Lett. 110 (2013) 063003

A fast quenching of the dissociation of anthracene cation is observed.

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patrick.rousseau@unicaen.fr EMIE-UP summer school “Multiscale Dynamics in Molecular Systems” 27

Fast radiative cooling: IR cooling rate: →! Poincaré fmuorescence

Fast radiative cooling of anthracene (2)

Decay as a function of the cooling time shows that the anthracene population cools down on the ms timescale. →! internal energy distribution

  • S. Martin et al.
  • Phys. Rev. Lett. 110 (2013) 063003
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patrick.rousseau@unicaen.fr EMIE-UP summer school “Multiscale Dynamics in Molecular Systems” 28

Recurrent fmuorescence : excited state

: decay in ms time range : decay below 0.1 ms. Poincaré fmuorescence requires excited states for inverse internal conversion.

  • G. Ito et al.
  • Phys. Rev. Lett. 112 (2014) 183001
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patrick.rousseau@unicaen.fr EMIE-UP summer school “Multiscale Dynamics in Molecular Systems” 29

Recurrent fmuorescence : photon detection

  • Y. Ebara et al.
  • Phys. Rev. Lett. 117 (2016) 133004

Detection of 607 nm photon correlated with revolution

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patrick.rousseau@unicaen.fr EMIE-UP summer school “Multiscale Dynamics in Molecular Systems” 30

Take home message #2

The emission of one photon allows to dissipate some excess energy. →! radiative cooling This may occur through both electronic and vibrational transitions. Vibrational transition:

  • IR photons
  • slow process (typically ms)

Electronic transition:

  • through inverse internal conversion
  • presence of excited states
  • recurrent fmuorescence of near-IR/visible photons
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patrick.rousseau@unicaen.fr EMIE-UP summer school “Multiscale Dynamics in Molecular Systems” 31

Intr trodu ducti ction Instru strume ment ntatio ion Me Metastable d disso sociation

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Radi diati tive c coo

  • oling

Vibrational electron detachment Slow iso isome merisa sati tion

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patrick.rousseau@unicaen.fr EMIE-UP summer school “Multiscale Dynamics in Molecular Systems” 32

Electron attachment to

The excess energy due to electron attachment can induce vibrational excitation: Further decays include:

  • dissociation (DEA)
  • radiative cooling
  • vibrational auto-detachment (VAD)
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patrick.rousseau@unicaen.fr EMIE-UP summer school “Multiscale Dynamics in Molecular Systems” 33

VAD of : rates

One may take into account the rotation of excited molecules. In order to observe the VAD, long storage time (several 100 ms) are required. →! cryogenic storage device

PhD of S. Merk Heidelberg University (2013)

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patrick.rousseau@unicaen.fr EMIE-UP summer school “Multiscale Dynamics in Molecular Systems” 34

Cryogenic electrostatic storage devices

Cryogenic environments allow to increase the lifetime of the stored beam.

Storage of at CSR MPIK webpage

CSR Heidelberg DESIREE Stockholm

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patrick.rousseau@unicaen.fr EMIE-UP summer school “Multiscale Dynamics in Molecular Systems” 35

Storage in CTF

Using a cryogenic EIBT it is possible to observe the VAD otherwise hidden by the depletion due to the background pressure. Model fjts well the data for difgerent source parameters.

  • S. Merk et al.
  • Phys. Rev. A 89 (2014) 022502
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patrick.rousseau@unicaen.fr EMIE-UP summer school “Multiscale Dynamics in Molecular Systems” 36

Decay of

Similarly to , metal dimer such as can decays through dissociation or VAD.

  • J. Fedor et al.
  • Phys. Rev. Lett. 94 (2005) 113201

Using room temperature storage device, the decay rate is well described (below 100 ms) by a power law assuming the fragmentation from high rotational levels. Decay at further time range is associated with the depletion of the beam due to collision with the residual gas.

1/signal (arb. units)

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patrick.rousseau@unicaen.fr EMIE-UP summer school “Multiscale Dynamics in Molecular Systems” 37

Decay of in cryogenic ring

Longer storage time are obtained with the DESIREE cryogenic ring. Deviation from the power law decay is not anymore due to depletion of the beam. →! contribution from the VAD

PhD of E. K. Anderson Stockholm University (2019)

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patrick.rousseau@unicaen.fr EMIE-UP summer school “Multiscale Dynamics in Molecular Systems” 38

VAD vs. dissociation of

Coincidences between detectors disentangle the two contributions.

PhD of E. K. Anderson Stockholm University (2019)

For : high J level →! fragmentation high vibrational levels →! VAD no J - v exchange

  • VDE = 1.06 eV
  • D0 = 1.37 eV
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patrick.rousseau@unicaen.fr EMIE-UP summer school “Multiscale Dynamics in Molecular Systems” 39

Take home message #3

Vibrational auto-detachment (VAD) →! transfer from the vibrational energy to the electronic excitation →! electron emission Timescale can be very long (ms range) →! VAD “hidden” by beam depletion →! need of cryogenic device As the radiative cooling, VAD may be used to determine the internal energy distribution.

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patrick.rousseau@unicaen.fr EMIE-UP summer school “Multiscale Dynamics in Molecular Systems” 40

Intr trodu ducti ction Instru strume ment ntatio ion Me Metastable d disso sociation

  • n

Radi diati tive c coo

  • oling

Vibrati tiona nal e electr ctron d detachm chment Slow isomerisation

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patrick.rousseau@unicaen.fr EMIE-UP summer school “Multiscale Dynamics in Molecular Systems” 41

Isomerisation of

Passage from linear isomer to cyclic isomer →! isomerisation barrier: 2.68 eV

  • K. Saha et al.

Nature Comm. 9 (2018) 912

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patrick.rousseau@unicaen.fr EMIE-UP summer school “Multiscale Dynamics in Molecular Systems” 42

Preparation of

Trapping:

  • vibrationnaly cold
  • some rotational excitation

Laser excitation:

  • after 190 ms
  • hν = 2.48 eV – 2.92 eV

→! below and above →! no VAD from linear

  • K. Saha et al.

Nature Comm. 9 (2018) 912

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patrick.rousseau@unicaen.fr EMIE-UP summer school “Multiscale Dynamics in Molecular Systems” 43

Competitive processes

VAD: IR radiative cooling: Recurrent fmuorescence from linear isomer: Isomerisation

  • K. Saha et al.

Nature Comm. 9 (2018) 912

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patrick.rousseau@unicaen.fr EMIE-UP summer school “Multiscale Dynamics in Molecular Systems” 44

Neutralisation rates

Neutralisation by VAD from linear isomer after 1 and 2-photon excitation Neutralisation of cyclic isomer after isomerisation For one-photon excitation: For two-photon excitation: Total neutralisation rate:

  • K. Saha et al.

Nature Comm. 9 (2018) 912

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patrick.rousseau@unicaen.fr EMIE-UP summer school “Multiscale Dynamics in Molecular Systems” 45

decay curves

Model fjts well the experimental data. Two-photon excited molecules promptly decay. One-photon excitation:

  • K. Saha et al.

Nature Comm. 9 (2018) 912

Slow isomerisation

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patrick.rousseau@unicaen.fr EMIE-UP summer school “Multiscale Dynamics in Molecular Systems” 46

Rate coeffjcients

Isomerisation is slow due to the barrier to pass BUT promptly after isomerisation, the cyclic isomer decays via VAD

  • K. Saha et al.

Nature Comm. 9 (2018) 912

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patrick.rousseau@unicaen.fr EMIE-UP summer school “Multiscale Dynamics in Molecular Systems” 47

Final take home message

Excited molecular system de-excites via various processes →! statistical physics Both electronic and vibrational dynamics observed in the range µs – s →! coupling between electronic and vibrational degrees of freedom (dissociation, isomerisation, radiative cooling, VAD) (Cryogenic) storage devices allow to prepare a well-defjned system

Action spectroscopy

  • f stored coronene cation
  • M. H. Stockett et al.

Faraday Discuss. 217 (2019) 126

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patrick.rousseau@unicaen.fr EMIE-UP summer school “Multiscale Dynamics in Molecular Systems” 48

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patrick.rousseau@unicaen.fr EMIE-UP summer school “Multiscale Dynamics in Molecular Systems” 49

Internal energy distribution

Photo-enhanced electron emission allows to determine the internal energy distribution.

  • M. Goto et al.
  • J. Chem. Phys. 139 (2015) 054306
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patrick.rousseau@unicaen.fr EMIE-UP summer school “Multiscale Dynamics in Molecular Systems” 50

Internal energy distribution (2)

Vertical auto-detachment can also give the internal energy distribution.

  • C. Breitenfeldt et al.
  • Phys. Rev. Lett. 120 (2018) 253001