Photochemical processes play an integral role in our day-to-day lives. Nature has carefully chosen our molecular

building blocks so that the potentially devastating effects of ultraviolet (UV) radiation absorption are by-passed. For

example, adenine readily absorbs UV radiation, however it’s spectroscopy is characterized by low fluorescence

quantum yields [1]. The high rate of non-radiative decay in this and other similar hetero-aromatic systems minimizes

the lifetimes of the lowest excited states and as a result reduces the risk of photochemical damage.

Ab initio [2] electronic structure calculations indicate that there may be a very simple radiationless decay pathway

that governs the high rates of non-radiative decay in aromatic and heterocyclic biomolecules. 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 both the initially excited 1ππ* state and the electronic ground state along

the 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. Since the prediction of the 1πσ* relaxation pathway by ab

initio calculations an increasing number of ultrafast, femtosecond experiments have been designed to “search” for

their spectroscopic evidence.

Figure 1: Molecular structures of some of the molecules of interest: Tyrosine (a), tryptophan (b) and adenine (c). The dissociative X-H bond attributed to 1πσ* is highlighted in yellow.

(c)Introduction

Experimental setup

Molecular beam

Laser beams

Velocity focusing lens

Phosphor screen

MCP detector

Drift tube

The optical setup uses a commercial femtosecond laser

system which is split into three beams of equal

intensity. The first beam is used to generate the 4th

harmonic (pump) at 200 nm through frequency

quadrupling. The remaining two beams are used to

pump two OPA’s. One of the outputs is set at 243.1 nm

to probe neutral H atoms via (2+1) REMPI. The

pump/probe beams are combined collinearly at a beam-

splitter and sent into the interaction region of a velocity

map ion imaging (VMI) spectrometer which intercepts a

molecular beam of sample. The molecular beam is

produced by seeding a vapour pressure of target

molecule in He, admitted into vacuum using an Even-

Lavie pulsed solenoid valve.

Source

chamberInteraction

chamber

Linear

TOF-MS VMI setup

Prototypical system-ammonia

Using a combination of pump/probe spectroscopy and VMI, we have

studied the multichannel photodissociation of ammonia with the

ultimate goal of carrying forward our knowledge of this simple

system to more complex biomolecules displaying similar non-

adiabatic dynamics [6]. We have been able to clock the real-time

dissociation of the N-H bond, following excitation of the ν’2 = 4

umbrella mode in the A-state (Fig. 4). In doing so, we have partially

untangled the various dissociation paths which lead to different final

states of the NH2 fragment (Figs. 5 and 6). Dissociation lifetimes of

the H-atoms with the least KEs suggests the onset of adiabatic

dissociation effectively competing with the non-adiabatic pathway. Figure 4: Cut through the A- and X-state potential curves in ammonia, taken from Bach et al. [5]

Figure 5: H+ transient as a function of H-atom KE.

H-atom kinetic energy / cm-1

H+ s

ign

al

t=0

Pump/probe (1 ps)

0 4000 8000 12000 16000

Probe/pump (0.25 ps)

+200 nm

with KE

0 2 4 6 8 10

v2 (NH2 X-state)

Life

tim

e (

τ)

/ fs

0

100

200

300

-0.5 0 0.5 1 1.5 2.0

t=59 fs(v2=1)

Time / ps

H+ s

ign

al

t=173 fs(v2=6)

t=250 fs(v2=9)

By probing the H-atoms following UV excitation at 200 nm, we recently showed that H-atom elimination

along the dissociative 1πσ* PES in phenol occurs within 103 ± 30 fs (Fig. 8) [7], indicative of very

efficient coupling at the S1/S2 and S0/S2 CI’s (Fig. 9 – orange path). This non-statistical route is in

competition with the statistical pathway (unimolecular decay) which was not observed on the timescale

of these measurements (< 200 ps).

Phenol-the chromophore of tyrosineSlo

w H

Fast

H

Phenol-h6

Phenol-d5

Work in progress.... References [1] Crespo-Hernàndez, C. E.; Cohen, B.; Hare, P. M.; Kohler, B. Chem. Rev. 2004, 104, 1977 [2] Sobolewski, A. L.; Domcke, W., Chem. Phys. 2000, 259, 181 [3] Eppink, A. T. J. B.; Parker, D. H., Rev. Sci. Instrum. 1997, 68, 3477 [4] Roberts, G. M; Nixon, J. L.; Lecointre, J.; Wrede, E.; Verlet, J. R. R., Rev. Sci. Instrum. 2009, 80, 053104 [5] Bach, A.; Hutchison, J. M.; Holiday, R. J.; Crim, F. F. J. Phys. Chem. A. 2003, 107, 10490 [6] Wells, K. L.; Perriam, G.; Stavros, V. G., J. Chem. Phys. 2009, 130, 074308 [7] Iqbal, A. J.; Pegg, L. G.; Stavros, V. G., J. Phys. Chem. A. 2008, 112, 9531 [8] Iqbal, A. J.; Cheung, M. S. Y.; Nix, M. G. D.; Stavros, V. G., J. Phys. Chem. A. 2009, 113, 8157 [9] Wells, K. L.; Roberts, G. A.; Stavros, V. G., Chem. Phys. Lett., 2007, 446, 20[10] Bisgaard, S.Z.; Satzger, H.; Ullrich, S.; Stolow, A., ChemPhysChem. 2009, 10, 101

Acknowledgements

The interaction chamber contains the VMI detector, replicating the setup by Eppink and Parker

to detect H-atoms [3]. A timed voltage pulse extracts H+ ions towards the detector consisting of

a 40 mm diameter Chevron microchannel plate assembly coupled to a phosphor screen

(Photek). Deconvolution of the raw images is done using the polar ion peeling method [4].

Velocity map ion imaging (VMI)

Statistical vs. non-statistical decay has been an area of increasing interest in the literature. Very recently [8],

we have shown using a combination of pump/probe spectroscopy and VMI, that H-atom elimination in phenol-

d5 giving low KE H-atoms occurs in < 150 fs (Figs. 10-central spot and 11 (a)), in sharp contrast to what one

expects from a statistical decay process, which would also give low KE H-atoms. This implies that these H-

atoms are formed through a non- statistical dynamic route and not through a statistical route as previously

thought.

Figure 6: Integrated H+ signal corresponding to NH2 fragments coming off with different quanta of ν2 (bend).

Figure 7: Observed lifetime (τ) vs. quanta of ν2 bend in NH2.

In our efforts to understand the role of the 1πσ* in the excited state dynamics of larger, more complex systems, our research is currently looking at

the photochemistry of imidazole (en route to adenine [9, 10]) and derivatives there-of in addition to tyrosine.

Figure 10: Raw images for H+ following photodissociation at 200 nm. Only half of each image is shown for illustrative purposes. The probe pulse is delayed 2 ps relative to the pump.

Figure 12: Extended H+ transients. Circles and squares represent high and low KE H-atoms.

Adenine

Imidazole part

Thanks go to Lara-Jane Pegg for experimental assistance. We would like to thank Dr Jan Verlet and Mr Gareth Roberts for use of their polar onion peeling program and discussions about VMI. We would also like to thank Dr Mike Nix for modelling the data and valuable discussions and Professor Mike Ashfold for valuable discussions.

Figure 9 (right): Cut along RO-H for the S0, S1 and S2 PES of

phenol-h6

-1 -0.5 0 0.5 1 1.5 2 2.5

H+

signal

Time / ps

1

103 fs±30 fs

0

Data: Data1_B

Figure 8 (left): Total integrated H+ transient as a function of pump (200 nm)/probe(243.1 nm) delay

Figure 11: H+ transients as a function of pump (200 nm)/probe (243.1 nm) delay for the low and high KE H-atoms in phenol-d5.

Figure 3: LHS shows a schematic of the VMI setup. RHS depicts a raw image of H+ ions following photodissociation of HBr at 200 nm and REMPI of the H-atoms with 243.1 nm. The dominant channel at 200 nm gives Br.

Figure 2: Schematic of experimental setup (RHS) and a picture of our molecular beam machine (LHS).

Dynamics of X-H dissociation in biomolecules

Imidazole 2-methyl imidazole 4-methyl imidazole

(a)

(b)

Kym Wells, Azhar Janjuah, Gareth Perriam, Michelle Cheung, David Hadden and Vasilios Stavros

Department of Chemistry, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK

Br* + H (β0)

Br + H (β-1)

HBr 200 nm Br

Br*

Dissociation leaves the NH2 excited vibrationally, the extent of excitation determining the KE in the H-atom fragment.

Indole part

Tryptophan

t=90 fs (imidazole)

t=140 fs (4-methylimidazole)

t=160 fs (2-methylimidazole)

-0.5 0 0.5 1 1.5 2.0 2.5 Time / ps

H+ s

ign

al

Indole

Financial support

Figure 15: Raw half-images for H+ following photodissociation of indole at 200 nm. The probe pulse is delayed (as indicated) relative to the pump pulse.

Figure 13 (top) : Total integrated H+ transient as a function of pump (200 nm)/probe(243.1 nm) delay for imidazole and its derivativesFigure 14 (bot): Cut along RN-H for S0 and S1 PES of imidazole/derivatives

243 nm

200 nm

4

5

6

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

RO-H AO

PE /

eV

imidazole

2,4-methylimidazole2-methylimidazole4-methylimidazole