This document describes an investigation of the excited states of indole using time-resolved photoelectron spectroscopy and time-resolved kinetic energy release measurements. The study probes indole with photons between 241-273 nm and monitors fragmentation and H-atom dissociation to analyze the 1ππ* and 1πσ* excited states. The results provide relaxation time constants for the excited states that are consistent between techniques and with previous studies. The work aims to resolve discrepancies in reported timescales and understand the photodynamics of indole, which could inform studies of biologically relevant molecules like eumelanin.

1. A Wavelength Dependent Investigation of the 1ππ* and 1πσ* States in Indole via
Photoionization and Fragmentation Pump-Probe Spectroscopy
T. J. GODFREY, Hui Yu, Michael S. Biddle, and Susanne Ullrich
Department of Physics and Astronomy, Physics Building, University of Georgia, Athens, GA 30602
E-mail: godfrey@uga.edu; ullrich@physast.uga.edu
TR-PES
EXPERIMENT
TR-IY:
 Indole probed with 294 nm photons.
 Mass and excited-state fragmentation
information aids in TR-KER and TR-
PES analysis.
TR-KER:
 H-atoms probed at 243.15 nm.
 Detects H-atoms using magnetic bottle
spectrometer.
 Time-of-flight (TOF) measurements are
converted to H-atom kinetic energy.
 Provides indirect analysis of 1πσ* state.
TR-PES:
 Indole probed with 294 nm photons.
 Photoelectron signal is deconvoluted
using a Levenberg-Marquardt optimi-
zation algorithm.
 Provides direct analysis of all excited
states in relaxation process.
 TOF measurements converted to photo-
electron kinetic energy with 1,3-buta-
diene and ammonia.
CONCLUSIONS REFERENCES
GENERAL:
 Molecular beam generated by heating
indole to 35 °C. Indole vapor carried by
helium gas.
 Indole pumped at wavelengths in the
241-273 nm range.
TR-KER
m/z = 117
m/z = 116
λpu
(nm)
λpr
(nm)
1La-State
Activity
1Lb-State
Activity
1πσ*-State
Activity
200[1] 243.15 - - 100 ± 30 fs
201[2] 294 <100 fs 23 ± 5 ps 405 ± 76 fs
201[2] 243.15 - - 367 ± 39 fs
201[2] 294 <100 fs 23 ± 5 ps 385 ± 69 fs
235[3] 1305 - 150 ± 20 ps -
243[3] 1305 22 ± 9 fs 315 ± 50 ps 435 ± 125 fs
248[3] 1305 22 ± 9 fs ps 460 ± 145 fs
249[4] 300 <100 fs 350 ps (±20%) 700 fs (±20%)
260[3] 1305 22 ± 9 fs 7 ± 2 ns 370 ± 110 fs
269[3] 1305 39 ± 12 fs ns -
272.5[3] 1305 42 ± 9 fs ns -
273[4] 300 <100 fs  1200 fs (±20%)
278[3] 1305 - ns -
Several works [Refs. 5-9] demonstrate that information acquired from gas-phase
experiments can have direct applications in condensed-phase studies; hence, gas-phase
spectroscopic work such as this could be a significant first step in understanding the photo-
dynamics of biologically relevant molecules, like indole, in the condensed phase.
TABLE 1. Reported relaxation times associated with
1La-state, 1Lb-state, and 1πσ*-state dynamics.
FIGURE 1. H-atom signal
associated with 1πσ*-state
relaxation dynamics.
Results SummaryResults Summary
FIGURE 2. Color maps representing the
individual channel contributions are dis-
played here. Photoelectrons originating
from the 1La, 1Lb, and 1πσ* states are
presented in columns one, two, and three,
respectively.
FIGURE 3. Delay traces (○), fits (red)
individual channel contributions (shown
in green, purple, and black), and the
extracted decay times at all pump
wavelengths are displayed here.
 High-energy H-atom signal is
associated with 1πσ*-state ac-
tivity.
 H-atom dissociation from N-
H bond starts in low- to mid-
260 nm range.
 The inversely related trend
between pump-photon energy
and rise time suggests a po-
tential energy barrier for 1πσ*
state.
 Bimodal H-atom distribution
suggests at least two possible
pathways leading to H-atom
generation.
 Low-energy H-atom signal
linked to statistical processes.
 Results highly consistent with
TR-PES investigation.
 Three excited-state channels need-
ed to accurately fit signal at 241,
250, and 260 nm.
 Two excited-state channels needed
to fit data at 270 and 273 nm.
 1La-state signal displays trend sug-
gestive of a potential energy bar-
rier.
 Signal originating from 1πσ* state
exhibits time constants very similar
to TR-KER results.
 Signal correlation with first two
ionization potentials of 7.90 eV and
8.35 eV (labeled IP1 and IP2 in
FIGURE 2) is highly consistent.
 No evidence of 1πσ*-state relax-
ation at 270 and 273 nm.
 TR-IY results reveal little to no
fragmentation in indole and ex-
tracted time constants are similar to
those in FIGURE 3.
[1] A. Iqbal and V. G. Stavros, J. Phys. Chem. A 114, 68 (2010).
[2] T. J. Godfrey, H. Yu, and S. Ullrich, J. Chem. Phys. 141, 044314 (2014).
[3] R. Montero, A. P. Conde, V. Ovejas, F. Castano, and A. Longarte, J. Phys. Chem. A 116,
2698 (2012).
[4] R. Livingstone, O. Schalk, A. E. Boguslavskiy, G. R. Wu, L. T. Bergendahl, A. Stolow, M. J.
Paterson, and D. Townsend, J. Chem. Phys. 135, 194307 (2011).
[5] S. J. Harris, D. Murdock, Y. Zhang, T. A. A. Oliver, M. P. Grubb, A. J. Orr-Ewing, G. M. Greetham,
I. P. Clark, M. Towrie, S. E. Bradforth, and M. N. R. Ashfold, Phys.Chem. Chem. Phys. 15, 6567
(2013).
[6] Y. Zhang, T. A. A. Oliver, M. N. R. Ashfold, and S. E. Bradforth, Faraday Discuss. 157, 141 (2012).
[7] D. Murdock, S. J. Harris, T. N. V. Karsili, G. M. Greetham, I. P. Clark, M. Towrie, A. J. Orr−Ewing,
and M. N. R. Ashfold, J. Phys. Chem. Lett. 3, 3715 (2012).
[8] T. A. A. Oliver, Y. Zhang, M. N. R. Ashfold, and S. E. Bradforth, Faraday Discuss. 150, 439 (2011).
[9] F. F. Crim, Faraday Discuss. 157, 9 (2012).
 Observed relaxation dynamics are consistent
with the potential energy profiles in FIG-
URE 4.
 Many decay times presented here contradict
previous reports (especially those related to
1La-state activity).
 No significant evidence is found to suggest
1πσ* activity at wavelengths greater than
approximately 263 nm. FIGURE 4. Indole potential energy cuts.[2]
INTRODUCTION
MOTIVATION
Procuring an exhaustive and consistent understanding of the aromatic
heterocyclic molecule indole is becoming immensely important as its
application in biological processes becomes more evident. As a
building block of the eumelanin polymer (pictured to the left),
understanding indole photodynamics following excitation in the
ultraviolet (UV) region may expand our overall comprehension of
eumelanin relaxation mechanisms.
TABLE 1 is a summary of the
reported indole relaxation times gleaned
at various pump (column 1) and probe
(column 2) wavelengths related to
specific excited states (columns 3-4).
Clearly, significant disparities still exist.
It is the aim of this work to study the
excited-state dynamics of indole through
a wavelength dependent study within the
UV region where the 1πσ*-state onset,
which is under current debate, is located
and to evaluate all relaxation pathways.
Associated time constants are also
extracted and reported using three
experimental techniques.
Indole