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 UllrichDepartment 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 excitedstates 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 byhelium gas.

Indole pumped at wavelengths in the241-273 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 fs201[2] 294 <100 fs 23 ± 5 ps 405 ± 76 fs201[2] 243.15 - - 367 ± 39 fs201[2] 294 <100 fs 23 ± 5 ps 385 ± 69 fs235[3] 1305 - 150 ± 20 ps -243[3] 1305 22 ± 9 fs 315 ± 50 ps 435 ± 125 fs248[3] 1305 22 ± 9 fs ps 460 ± 145 fs249[4] 300 <100 fs 350 ps (±20%) 700 fs (±20%)260[3] 1305 22 ± 9 fs 7 ± 2 ns 370 ± 110 fs269[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-phaseexperiments can have direct applications in condensed-phase studies; hence, gas-phasespectroscopic 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 Summary Results Summary

FIGURE 2. Color maps representing theindividual channel contributions are dis-played here. Photoelectrons originatingfrom the 1La, 1Lb, and 1πσ* states arepresented in columns one, two, and three,respectively.

FIGURE 3. Delay traces (○), fits (red)individual channel contributions (shownin green, purple, and black), and theextracted decay times at all pumpwavelengths are displayed here.

High-energy H-atom signal isassociated with 1πσ*-state ac-tivity.

H-atom dissociation from N-H bond starts in low- to mid-260 nm range.

The inversely related trendbetween pump-photon energyand rise time suggests a po-tential energy barrier for 1πσ*state.

Bimodal H-atom distributionsuggests at least two possiblepathways leading to H-atomgeneration.

Low-energy H-atom signallinked to statistical processes.

Results highly consistent withTR-PES investigation.

Three excited-state channels need-ed to accurately fit signal at 241,250, and 260 nm.

Two excited-state channels neededto fit data at 270 and 273 nm.

1La-state signal displays trend sug-gestive of a potential energy bar-rier.

Signal originating from 1πσ* stateexhibits time constants very similarto TR-KER results.

Signal correlation with first twoionization potentials of 7.90 eV and8.35 eV (labeled IP1 and IP2 inFIGURE 2) is highly consistent.

No evidence of 1πσ*-state relax-ation at 270 and 273 nm.

TR-IY results reveal little to nofragmentation in indole and ex-tracted time constants are similar tothose 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,

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[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 consistentwith the potential energy profiles in FIG-URE 4.

Many decay times presented here contradictprevious reports (especially those related to1La-state activity).

No significant evidence is found to suggest1πσ* activity at wavelengths greater thanapproximately 263 nm. FIGURE 4. Indole potential energy cuts.[2]

INTRODUCTION

MOTIVATIONProcuring an exhaustive and consistent understanding of the aromaticheterocyclic molecule indole is becoming immensely important as itsapplication in biological processes becomes more evident. As abuilding block of the eumelanin polymer (pictured to the left),understanding indole photodynamics following excitation in theultraviolet (UV) region may expand our overall comprehension ofeumelanin relaxation mechanisms.

TABLE 1 is a summary of thereported indole relaxation times gleanedat various pump (column 1) and probe(column 2) wavelengths related tospecific excited states (columns 3-4).Clearly, significant disparities still exist.It is the aim of this work to study theexcited-state dynamics of indole througha wavelength dependent study within theUV region where the 1πσ*-state onset,which is under current debate, is locatedand to evaluate all relaxation pathways.Associated time constants are alsoextracted and reported using threeexperimental techniques.

Indole