DNA photoprotection• Strong absorption in the UV• Photo excited states reactive: How does the molecule survive?• Predicted non-Born-Oppenheimer

relaxation: 100 fs? Several ps?• New approach gives spatial/orbital sensitivity

Experiment:

Results: Future:

AugerMatrix Element:

p

n

IP

O1s

p*UV Excitation Photoprotection

n → p p → p*

X-Ray Probe

X-ra

y pu

lse

Auger

1 2

3

Δt?TimeDelay

baba

corefree r

,

1

Valence and core overlap give site and orbital selectivity

n orbital contributions

Core

Motivation:

ExcitationTu

nabl

e X-

Ray

Prob

e

• Tunable/Selectable Photon Energy: Monochrometer/Seeded FEL. Desired Resolution: < 0.5 eV• Other DNA bases/ Gas phase samples• Liquid samples/environments

Pump ON – Pump OFFPump ON+Pump OFF

Time dependent X-Ray Absorption and Emission Measurements

Time Independent NEXAFS: ALS BL8

Capabilities:Electron Kinetic Energy (eV)

490 500 510 520 530 540

Phot

on E

nerg

y (e

V)

528

530

532

534

536

538

540

Femtosecond Dynamics in the Photoprotection in ThymineBrian McFarland and the LCLS Nucleobase Photoprotection collaboration