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Author(s): David J. Hadden, Craig A. Williams, Gareth M. Roberts and
Vasilios G. Stavros
Article Title: Time-resolved velocity map imaging of methyl elimination
from photoexcited anisole
Year of publication: 2011
Link to published article: http://dx.doi.org/10.1039/C0CP02429E
Publisher statement: None
Time resolved velocity map imaging of methyl elimination
from photoexcited anisole
David J. Hadden, Craig A. Williams, G.M. Roberts and Vasilios G. Stavros*
Department of Chemistry, University of Warwick, Coventry, CV4 7AL, UK
*To whom correspondence should be addressed. E-mail: firstname.lastname@example.org.
To date, H-atom elimination from heteroaromatic molecules following UV excitation
has been extensively studied, with the focus on key biological molecules such as
chromophores of DNA bases and amino acids. Extending these studies to look at
elimination of other non-hydride photoproducts is essential in creating a more
complete picture of the photochemistry of these biomolecules in the gas-phase. To
this effect, CH3 elimination in anisole has been studied using time resolved velocity
map imaging (TR-VMI) for the first time, providing both time and energy information
on the dynamics following photoexcitation at 200 nm. The extra dimension of energy
afforded by these measurements has enabled us to address the role of πσ* states in the
excited state dynamics of anisole as compared to the hydride counterpart (phenol),
providing strong evidence to suggest that only CH3 fragments eliminated with high
kinetic energy are due to direct dissociation involving a
1πσ* state. These
measurements also suggest that indirect mechanisms such as statistical unimolecular
decay could be contributing to the dynamics at much longer times.
Photochemistry is everywhere, playing a vitally important role in our day-to-
day lives. For example our molecular building blocks readily absorb ultraviolet
radiation. However these molecules display a large degree of photostability.1-3 One of
the main reasons for this is that photochemical reactions are effectively quenched
through ultrafast non-radiative processes imparting a high degree of photostability to
these building blocks of life.3-5 In light of this, there have been growing efforts both
from an experimental and theoretical standpoint, aimed at precisely defining
photoresistive pathways potentially operative in some of nature’s most important
molecules. Recently, dissociative 1πσ* states have been implicated as key players in
the photoresistive properties of chromophores of aromatic amino acids and DNA
bases.5 Whilst a number of experiments have been directed at searching for
spectroscopic evidence of 1πσ* states with some success on smaller molecules (e.g.
phenol6-8 and indole,9-11 the chromophores of the amino acids tyrosine and tryptophan
and DNA bases such as adenine12-14), the ever-burgeoning question is to what extent
do 1πσ* states contribute to the photochemistry of a larger range of biological
The seminal theoretical work of Domcke and Sobolewski originally suggested
a general decay pathway that may be common to a number of aromatic and
heterocyclic molecules.5,15 Upon UV irradiation, photon energy is deposited into the
molecule through excitation to an optically bright 1ππ* state. In these molecules, an
excited state of 1πσ* character intersects, through conical intersections (CIs), both the
initially excited 1ππ* state and the electronic ground state along an X-H stretch
coordinate where X is typically O or N. Non-radiative decay along this pathway is
predicted to be highly efficient due to the repulsive nature of the 1πσ* state leading to
H-atom elimination. Since the prediction of the 1πσ* relaxation pathway by ab initio
calculations an increasing number of experiments have been carried out in the gas
phase given the comparison with existing theory which has thus far largely modelled
isolated molecules. The 1πσ* states, however, are optically dark and their potential
energy surfaces (PESs) are dissociative along the X-H coordinate, making them
difficult to detect directly, except through time-resolved photoelectron spectroscopy.
This is often very challenging, especially as the complexity of the photoelectron
spectra increases with increasing molecular size.16 Instead, characteristics of these
states have been exploited for indirect observations. Spectroscopic detection of H-
atom appearance times and velocity distributions are indirect evidence for relaxation
along the 1πσ* state.17
Whilst a flurry of both experiment and theory has been directed towards 1πσ*
states in hydrides, in contrast much less effort has been directed at studying these
dissociative states localized on other coordinates such as X-C. The velocity map ion
imaging (VMI) work by Ashfold and co-workers was the first to identify the role of
1πσ* state induced bond dissociation in non-hydride heteroaromatic systems.18 Their
work on N-methylpyrrole clearly showed the bimodal distribution of CH3
photoproducts following excitation with UV radiation. In keeping with their H-atom
total kinetic energy release (TKER) spectra, the appearance of high and low kinetic
energy (KE) components in their spectra were attributed to direct dissociation along
the 1πσ* state resulting in high KE CH3 fragments and indirect dissociation, in which
highly excited ground state molecules decay, resulting in low KE CH3 fragments. This
work was further complemented by Becucci and co-workers whose combined
experimental and theoretical studies showed the existence of two low-lying
dissociative 1πσ* states localized on the X-CH3 coordinate in the same system.19
Using multi-mass ion-imaging, Ni and co-workers carried out a series of experiments
on N-methylindole, N-methylpyrrole and anisole, clearly showing the appearance of
either fast or slow CH3 (or both) fragments following photodissociation at 248 nm and
193 nm, to which they once again rationalized these findings by invoking the presence
of a 1πσ* state localized on the X-CH3 coordinate.20 Most recently, Lim and Kim
studied the photodissociation of thioanisole (C6H5S-CH3), observing a striking
dependence on the branching ratio between the ground and excited state C6H5S
fragment (X or A) reaction channels depending on excitation energy.21
Although these measurements have provided significant insight into the
underlying photophysics of these systems, with clear evidence for the active
participation of 1πσ* states, very little is known about the timescales of these
processes which is critical in unravelling the complex interplay between adiabatic and
non-adiabatic dynamics and factors which affect these. In previous studies in phenol-
h6 and phenol-d5, our group has shown that both the high and low KE components of
the H-atom KE release spectra indicated appearance timescales in the order of 100
femtoseconds.8 Whilst this timescale is unsurprising for a direct process in which
dissociation occurs within one vibrational period, what is surprising is the timescale
for the low KE component, given that this process is expected to be much longer, i.e.,
once on the electronic ground state, energy must be localized in the correct mode
before dissociation can occur (statistical unimolecular decay). This has led us to
suggest an alternative decay pathway to generate these low KE H-atoms which is
direct. In doing so, this has prompted the work presented here using time-resolved
velocity map ion imaging (TR-VMI) studies in anisole in order to compare and
contrast the dynamics of this system with its corresponding hydride counterpart (Fig.
1). The work presents strong evidence to suggest that following the same excitation
wavelength as in phenol-d5 (200 nm) the high KE component of the TKER spectra
imply appearance timescales for CH3 elimination that are ultrafast in contrast to the
low KE component. To the best of our knowledge, these are the very first time-
resolved measurements of CH3 elimination which implicate the participation of a 1πσ*
state, in agreement with recent multi-mass ion-imaging of the same system recorded
at 193 nm. Although still speculative, the results presented here also seem to suggest
that an indirect route to CH3 elimination may also be operative at much longer time-
delays (~102 ps), attributed to a much slower pathway, possibly statistical
For more details regarding the experimental setup, the reader is referred to an
earlier publication of ours.22 The experiments utilize a commercial femtosecond (fs)
laser system (Spectra-Physics XP) containing a Ti-Sapphire oscillator and a
regenerative amplifier. The amplifier operates at 125 Hz and is centred around 800
nm, delivering 35 fs pulses. The 800 nm output is split into three equally intense
beams. The 200 nm pump is obtained by frequency doubling 1 mJ of the fundamental
(BBO-type I) and then mixing the 400 nm with residual 800 nm (BBO-type II) to
generate 267 nm. The 267 nm is further mixed with residual 800 nm (BBO-type I) to
generate approximately 1 μJ/pulse of 200 nm. The remaining two beams are used to
pump two optical parametric amplifiers (TOPAS model 4/800/f, Light Conversion).
One of the outputs is tunable and is used to provide an alternative, variable-
wavelength pump (235 nm – 250 nm) while the other is set at 333.5 nm to probe
neutral CH3-atoms via (2+1) resonance-enhance multi-photon ionization using the
band of the 2-photon
2 ' '
transition,23 with an output power around 7
The optical delay between the pump and probe is varied over 100 picoseconds
(ps) with a minimum step size of 0.025 ps, controlled by a delay stage (Physik-
Instrumente). The instrument response function is 170 fs full-width at half-maximum
(FWHM), measured through multi-photon ionization of NH3 and Xe. This is also used
to determine time zero (t = 0) for the experiment. A collinear beam of pump and
probe pulses is obtained with the help of a dichroic mirror which is focused with the
aid of a 500 mm magnesium fluoride lens into the interaction region of a VMI
spectrometer to intercept a molecular beam of anisole. The molecular beam is
generated by seeding a vapour pressure of anisole (Sigma-Aldrich, ≥ 98 %) molecules
in He (2-3 atm. and 60 oC) and is sent into vacuum using an Even-Lavie pulsed
solenoid valve24 operating at 125 Hz and synchronized to the laser system. Typical
opening times of this valve are set to 10-15 μs.
The molecular beam machine consists of a source chamber and interaction
chamber, separated by a 2 mm skimmer. The source chamber houses the pulsed valve
while the interaction chamber contains the VMI detector, replicating the setup as
described by Eppink and Parker,25 to detect the neutral CH3 radicals. The CH3+ ions
are extracted towards the detector by a series of ion optics. The detector consists of a
40 mm diameter Chevron microchannel plate (MCP) assembly coupled to a P-43
phosphor screen (Photek). By applying a timed voltage pulse (Behlke) on the second
MCP, we are able to gate on a particular mass ion and record a 2-D CH3+ projection
by measuring the light emitted from the phosphor screen on a CCD array. The KE
spectrum of CH3-radicals is obtained after the deconvolution of the raw images using
an acquisition programme written in LabVIEW implementing the polar onion peeling
method.26 By measuring the current output directly from the phosphor screen, the
setup becomes a time-of-flight mass spectrometer. This enables us to reduce the
appearance of clusters by optimizing parameters such as the backing pressure,
opening time of the pulsed valve and delay between gas and laser pulses, resulting in
an observable reduction in the ion signal of anisole clusters (dimers and trimers etc.)
relative to the ion signal of the monomer in the time-of-flight mass spectrum.
III. Results and Discussion
Fig. 2a and 2b shows raw images of CH3+ at two pump/probe delays. Fig. 2a
corresponds to a delay (t) between the pump (200 nm) and probe (333.5 nm) pulses of
+1.5 ps, where the pump precedes the probe while Fig. 2b corresponds to a
pump/probe delay of -1.5 ps, the probe now preceding the pump. When the probe
precedes the pump, there is considerably less CH3+ signal as compared to when the
pump precedes the probe (approximately 10 times less total CH3+ signal), indicative
of a two-colour pump/probe signal in comparison to a combined pump alone and
probe alone signal at negative delays. The KE of the CH3 fragment following
dissociation is reflected in the distance of the corresponding CH3+ from the centre of
the image. In Fig. 2a, there is a small rise in the CH3+ signal around 180 pixels from
the centre of the image (or 18000 cm-1) that we attribute to CH3 formed through
dissociation via the 1πσ* state localized along the O-CH3 coordinate and is more
noticeable in the TKER spectrum shown in Fig. 3 (solid line). The origin of both high
and low KE components of the TKER spectrum forms the central discussion of the
present work, rationalized in the proceeding paragraphs.
Figure 3 shows the TKER distributions derived from deconvolution of the raw
CH3+ images and assuming C6H5O as the partner fragment, following photoexcitation
at 200 nm and probing through multiphoton ionization at 333.5 nm and 322.5 nm,
solid and dashed lines respectively. The delay between the pump and probe pulses
was set at 1.5 ps. The spectra are dominated by a low KE component which extends
towards high KE. On close inspection of the on-resonance TKER distribution (solid
line) one is able to discern a high KE component embedded within the tail of the low
KE component, manifested by the noticeable rise in the signal around 18000 cm-1,
shaded for clarity. When detuning from the 333.5 nm resonance, the high KE
component is significantly reduced, an indication of probing neutral CH3 through 2+1
REMPI with 333.5 nm following photodissociation.
In recent work on phenol-d5, which looks at H-atoms eliminated from the O-H
group alone,8 there is a clear bimodal distribution between the low and the high KE
components with each component sharing almost equal intensity. In contrast, the on-
resonance TKER distribution in Fig. 3 shows a much greater contribution of low KE
component as compared to the high KE counterpart. In addition, the high KE
component in anisole appears at higher values than the corresponding high KE
component in phenol-d5 following photoexcitation at 200 nm. This is unsurprising
given the decrease in the O-CH3 bond energy (~22500 cm-1) as compared to the O-H
bond energy in phenol (~31000 cm-1).20 The TKER distribution for the on-resonance
excitation shown here is in reasonable agreement to that previously reported by Ni
and co-workers, the high KE feature peaking in their study ~17000 cm-1 compared to
~18000 cm-1 measured here. We can estimate the O-CH3 bond energy from the
maximum TKER of the on-resonance distribution in Fig. 3. This value corresponds to
25000 - 26000 cm-1 implying the O-CH3 bond energy is 25000 - 24000 cm-1. This
compares reasonably well with the literature value of ~22500 cm-1, the difference
likely attributed to the limited resolution of our VMI spectrometer (~1500 cm-1 at
these energies) and assuming the phenoxyl radical is formed in its ground vibrational
From the on-resonance TKER distribution shown in Fig. 3, it seems unlikely
that we are forming both ground state and electronically excited C6H5O radicals. The
A-state of C6H5O lies ~8900 cm-1 above the ground state and as such, one would
anticipate a difference between the low KE and high KE components to be around this
value. The measurements in phenol-d5 showed this to be so, with the low and high KE
components separated by ~9000 cm-1. In anisole, the low and high KE components
are separated by ~17500 cm-1, the low KE component peaking around 400 cm-1. One
would therefore anticipate an additional peak at ~9000 cm-1 (18000 cm-1 – 8900 cm-1)
which is not immediately apparent in Fig. 3 unless this is buried beneath the tail of the
low KE component. As such, the low KE component in anisole peaking at 400 cm-1
cannot be attributed to CH3 with the partner C6H5O in the A-state.
Figures 4 and 5 show the first real-time CH3 elimination in anisole. The
transients are shown as a function of pump-probe delay in Fig. 4a, 4b and Fig. 5a, 5b
in which the probe was set at 333.5 nm (4a/5a) and 322.5 nm (4b/5b) with insets
displaying extended time delays. All four transients are obtained by collecting a series
of TKER spectra at various pump-probe delays (t) and integrating each TKER
spectrum around the low and high KE features; 230 – 5200 cm-1 and 11,500 - 23,000
cm-1 respectively for both probe wavelengths. Whilst the dynamics are insensitive to
the size of the spectral window in the high KE region (i.e. between 11,500 and 23,000
cm-1), we have chosen a large spectral window for the high KE component for
appreciable signal-to-noise. Perhaps the most notable difference in the four transients
is the step-like growth of the on-resonance high KE component (Fig. 4a) whilst in the
off-resonance high KE component (Fig. 4b) and low KE component (Figs. 5a and 5b),
the signal rises around t = 0 and then decays.
With these measurements, our aim is to obtain appearance timescales for CH3
elimination and in doing so provide us with detailed information regarding the
underlying photochemistry. Most pertinent to this study is whether dissociation occurs
directly along the 1πσ* state or through some statistical unimolecular decay process
on the electronic ground state. From Fig. 3, it is evident that the TKER distribution of
the on-resonance high KE component will contain both an on-resonance and off-
resonance contribution. As a result, any fitting of the corresponding on-resonance
transient shown in Fig. 4a must reflect this. The off-resonance transient shown in Fig.
4b (probe set at 322.5 nm) requires an exponential decay and step function to fit the
data (solid blue line) with lifetimes τORdecay < 65 fs and τORstep < 50 fs, where OR
denotes off-resonance. We are unable to quote precise values for τORdecay and τORstep
given that these timescales are at the limit of our temporal resolution, resulting in
large uncertainties in the time-constants. However, both processes are very fast and
are likely due to multiphoton excitations that generate CH3+ directly, such as: (1)
dissociative ionization of the parent ion through a short lived intermediate state
accessed by the pump (fast decay) and; (2) pump generated anisole+ (or associated
fragment) which is further excited by the probe undergoing dissociative ionization to
yield CH3+ (fast step).
To fit the on-resonance transient, we have used a combination of the function
obtained from the off-resonance transient with an additional step function, to yield a
time-constant of τR = 91 ± 36 fs, where R denotes on-resonance. Interestingly, this
time-constant is similar to that observed in the hydride counterpart (phenol) which our
group measured as τR = 88 ± 30 fs and is very likely indicative of direct dissociation
along the dissociative 1πσ* state. The two components of the fit, i.e. the off-resonance
component and the step function are shown in Fig. 4a by the blue and red lines
respectively. To cross-check the validity of this approach, the amplitudes of the off-
resonance component and step function (approximately 1:3 respectively) are in very
good agreement with the TKER spectra shown in Fig. 3 - multiplying the high KE
component of the off-resonance TKER spectrum by approximately 3 almost overlays
the two spectra in the high KE region.
The appearance time of τR = 91± 36 fs compared to the hydride counterpart of
τR = 88 ± 30 fs is much faster than one would anticipate based on the differences in
reduced masses of the two systems and peak positions of the high KE features,
~12000 cm-1 and ~18000 cm-1 in phenol and anisole respectively. Indeed, this would
correspond to an almost 3-fold difference between H and CH3 elimination, i.e. ~ 260
fs for CH3 elimination. However, one very important factor which determines the
time-constant (appearance time) of the fragment being probed is the minimum
internuclear separation upon which the fragment can be ionized and detected.
Unfortunately, these measurements are unable to determine the internuclear
separation beyond which one is able to detect CH3 (or H) fragments.
Interestingly the low KE transients at the two probe wavelengths are almost
identical in comparison to the high KE transients. Figs. 5a and 5b compare the
dynamics at probe wavelengths of 333.5 nm and 322.5 nm respectively following
excitation at 200 nm. In both cases, the transients are dominated by a rise in the signal
close to t = 0 followed by a decay to an elevated baseline. The low KE transients have
been obtained by integrating the low KE component (230 – 5200 cm-1) in both the on-
resonance and off-resonance KE spectra. It is tempting to fit the low KE CH3+
transients with an exponential decay and step function, in much the same way to the
off-resonance high KE component (see Fig. 4b). However, by extending the transients
at the two probe wavelengths to longer time-delays, as shown in the insets of Figs. 5a
and 5b, there is a clear decay. As a result, in much the same way to the high KE
components described above, we have fitted the off-resonance transient shown in Fig.
5b with an exponential decay and step function with lifetimes τORdecay1 = 85 ± 15 fs
and τORstep < 50 fs respectively and a further exponential decay with a lifetime of
τORdecay2 = 5.9 ± 0.7 ps obtained from the extended transient shown in the inset of Fig.
5b, the subscripts 1 and 2 representing the short and long decay functions
respectively. This combined function (solid blue line) is then used to fit the data
obtained in the on-resonance low KE transient, keeping the time-constants identical
(Fig. 5a). As evident, the fit is very good suggesting that the dynamics of both the on-
resonance and off-resonance low KE components, at least at short times, are very
similar, if not identical.
We are unable to quantify the exact mechanisms underlying the low KE
transient, however this is very likely multicomponent in nature, probably consisting of
a multiphoton part giving CH3+ directly through dissociative ionization, as evidenced
once again by the appearance of CH3+ off-resonance. Interestingly two decay
components are present, one with a short lifetime (~85 fs) and one with a longer
lifetime (5.9 ps - more visible in Fig. 5b) implying that the decay is occurring
sequentially through two states, one short lived, the other longer lived. Indeed a decay
of 5.9 ps is measured when probing the anisole+ transient, which seems to suggest that
this decay is from a short-lived intermediate state in the photoexcited anisole.
Whilst statistical unimolecular decay has thus far been ruled out in the short
time-transients (0 - 1.5 ps), it is clearly evident that for both the on-resonance low and
high KE transients collected at long time-delays up to 100 ps (insets of Fig. 4a and
Fig. 5a respectively), there is a slow rise in the CH3+ signal. Such behavior is absent,
within the signal-to-noise, in the off resonance transients (insets of Fig. 4b and Fig. 5b
respectively) and is suggestive that CH3 radicals are being generated through an
indirect process such as statistical unimolecular decay following population of the
ground state via internal conversion. This may either be through a conical intersection
between 1πσ*/S0 such as that suggested in the hydride counterpart27 or some other
non-radiative pathway. Alternatively, CH3 radicals could be formed through
fragmentation of the parent anisole+ or highly excited fragments thereof. It is clear
from these insets that the timescale for this process is likely to be very long. However
we are unable to determine the time-constant of this process with the current setup.
Using TR-VMI, CH3 radicals are eliminated with a range of KEs following
excitation of anisole at 200 nm. The TKER spectrum is dominated by a low KE peak,
with a small high KE feature centred around 18000 cm-1. Analysis of the CH3+
transients indicates that both the low and high KE components show dynamics on an
ultrafast timescale. The dynamics can be modelled using a step function for the high
KE component which can be accounted for assuming direct dissociation along the
1πσ* state with respect to the O-CH3 coordinate. The low KE component is likely due
to multiphoton processes dominating the dynamics such as dissociative ionization of
the parent cation following population of a long-lived state of the parent anisole.
The generalized model for X-H dissociation as suggested by Domcke and
Sobolewski seems to apply in non-hydride systems involving a repulsive 1πσ* state
following photoexcitation with UV light. The timescale for dissociation along this
state has been measured as τR = 91 ± 36 fs for CH3 elimination compared to τR = 88 ±
30 fs for H-atom elimination in phenol. One would anticipate an almost 3-fold
difference in the elimination time-scales for the two processes and possibly highlights
the point at which CH3 versus the H can be probed along the O-CH3 or O-H reaction
coordinate respectively. Answers to these questions will no doubt benefit from
electronic structure calculations on methylated counterparts in an attempt to correlate
the measured dynamics with those predicted from theory.
The authors gratefully thank Mr Nicholas Harding for experimental assistance
and helpful discussions and Dr Jan Verlet for use of his polar onion peeling program
and valuable discussions about VMI. The authors would also like to than Dr Mike Nix
for helpful discussions. D.J.H and C.A.W thank the EPSRC for doctoral research
fellowships. G.M.R thanks the Leverhulme Trust for postdoctoral funding. V.G.S
would like to thank the EPSRC for equipment grants (EP/E011187 and EP/H003401),
the Royal Society for a University Research Fellowship and the University of
Warwick for an RDF Award.
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Molecular structures of a) anisole and b) phenol
Raw images for CH3+ following photodissociation of anisole at 200 nm and probing
with 333.5 nm. The pump/probe delay was set to +1.5 ps and -1.5 ps in a) and b)
TKER spectra derived from deconvolution of raw CH3+ images and assuming C6H5O
as the partner fragment, following photoexcitation with 200 nm and probing with a)
333.5 nm (on-resonance) and b) 322.5 nm (off-resonance) radiation. The pump/probe
delay was set at 1.5 ps.
CH3+ transients as a function of pump (200 nm)/probe delay for high KE CH3
molecules probed using a) 333.5 nm and b) 322.5 nm respectively. At negative delays,
there is no appreciable 2-colour signal. Experimental data in a) and b) were fitted with
an off resonance decay and step function with lifetimes τORdecay < 65 fs and τORstep < 50
fs, respectively and an additional step function in a) with τR = 91 ± 36 fs, where OR
and R correspond to off-resonance and on-resonance respectively.
CH3+ transients as a function of pump (200 nm)/probe delay for low KE CH3
molecules probed using a) 333.5 nm and b) 322.5 nm respectively. Experimental data
in a) and b) were fitted with two off-resonance decay functions and a step function
having lifetimes of τORdecay1 = 85 ± 15 fs, τORdecay2 = 5.9 ± 0.7 ps and τORstep < 50 fs
TOC Figure and Caption:
TR-VMI measurements of methyl elimination from anisole indicate active
participation of a 1πσ* state in the photochemistry of anisole.