Preliminary Laboratory Studies of the Photoprocessing of PAH / H2O Mixtures in

the Interstellar Medium

John ThrowerDepartment of Chemistry, School of Engineering and Physical Sciences

Heriot Watt University, Riccarton, Edinburgh, EH14 4AS, UK

Polycyclic Aromatic Hydrocarbons (PAHs) Planar aromatic carbon networks May be origin of:

Unidentified Infra-Red emission bands (UIRs) Diffuse Interstellar Bands (DIBs)

No single PAH has been definitively identified in the ISM

Large PAHs may form part of the carbonaceous grain core population

Smaller PAHs expected to be present in icy mantles around interstellar grains H2O ice dominated environment

PAHs in the Interstellar Medium Some evidence for conversion to more complex

organics UV / ion irradiation → Photochemistry

Need to understand fundamentals – focus on desorption Simple model of PAH – C6H6

ISO – possible detection of C6H6

C6H6 less stable than larger PAHs Experimentally more convenient UV Spectrum in gas/liquid phases well known

Similar in the solid phase Solid state spectra obtained by Open University group at the

Daresbury Laboratory

Several possible channels following irradiation

UV Irradiation

Desorption following resonant absorption

Substrate Mediated Desorption

Photochemistry

The Experiment

New Ultrahigh Vacuum (UHV) system at Central Laser Facility Surface Science Techniques

LEED, AES, TPD, RAIRS Line of sight mass spectrometry (LoS-MS) Time of flight mass spectrometry (ToF-MS) Model the interstellar dust-grain interaction

Nanosecond pulsed lasers Induce desorption / photochemistry in model interstellar

ices

The Experiment

The Experiment Sapphire Substrate

Eliminate metal mediated effects Easily cooled to cryogenic temperatures

Held at UHV (<2×10-10 mbar) Substrate base temperature ~60-80 K

Closed cycle helium cryostat Ices deposited by introducing gases into chamber via

a fine leak valve – each layer = 200 L Irradiate at 248.8 nm (on-resonance), 250.0 nm

(near-resonance) and 275.0 nm (off-resonance) Laser powers: “low” (1.1 mJ/pulse) and “high” (1.8 mJ/pulse)

Sapphire Sapphire Sapphire Sapphire

C6H6

C6H6

C6H6H2O H2O

H2O

The Experiment

DyeLaserNd:YAG

QMS

MCS

trigger

30 40

0

500

1000

1500

Photon Induced Desorption Curves

Mas

s 78

SE

M c

ou

nts

/s

Time (s)

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

0

2

4

6

mcs

co

un

ts

time-of-flight (ms)

Photon induced desorption

Time of Flight (TOF)

Liquid N2

Benzene Desorption

More desorption “near-resonance” than “on-resonance”!

Desorption observed off-resonance from benzene absorption

250.0 nm feature “different”

Sapphire

C6H6

Benzene Desorption

More desorption at higher energies – expected as photon flux is increased. Suggests single photon process

Similar wavelength dependence and peak positions

Cannot use 1.8 mJ at 275 nm with current optics

Sapphire

C6H6

Benzene UV Absorption

Separate UV transmission experiments at Daresbury Laboratory by OU Group

Shift in peak position due to phase change between 60 K and 70 K

Temperature / K

Peak Position / nm

60 249.6

70 248.8

More absorption at 250.0 nm than 248.8 nm

Water Desorption

•Noisy due to higher water background

•Very little water desorption

•No strong wavelength dependence

Water does not absorb at these wavelength – any desorption must be substrate mediated

Sapphire

H2O

Benzene Desorption from Layered Systems

Less benzene desorbed when benzene is underneath water => benzene needs to diffuse through water

Other systems similar

Slight increase in benzene on water cf. benzene alone?

Water Desorption from Layered Systems

More desorption when benzene is present – energy transfer from benzene to water

Sharp feature on same timescale as benzene desorption

Very slow broad feature when water is on top of benzene – origin?

Maxwell-Boltzmann Fitting Fit to following Maxwell-Boltzmann function:

Where: t is the time of flight corrected for time between ionisation and

detection in QMS L is the physical distance from sample to point of ionisation T is effective temperature m is molecular mass k is the Boltzmann constant A is a scaling parameter

Only single Boltzmann component fitted – may need multiple components.

2

2

4

4

2expsignal TOF

t

L

kT

m

t

LA

Maxwell-Boltzmann Fitting

Desorption following resonant absorption by benzene produces “hotter” molecules than off-resonance.

On-resonance desorption is combination of substrate mediated and resonant effects

New non-absorbing substrate?

Sapphire

C6H6

Maxwell-Boltzmann Fitting

Benzene alone peak at similar temperature at high power

Benzene “cooler” when water is present

May be a difference in T between the two layering configurations?

All give rise to “hot” benzene

Conclusions Benzene and Water desorption strongest on-resonance with

benzene absorption at 250 nm Water desorption enhanced by presence of benzene

Energy transfer from benzene to water

Benzene comes off “hot”, i.e. Boltzmann temperature ~1000 K Astrophysical implications – highly energetic molecules

Evidence for “cooler” molecules following substrate mediated desorption (~500 K)

Overlayer of water reduces amount of benzene desorbed – but it still has T>900 K

Future Work Eliminate substrate mediated desorption channel

Study only pure resonance effects Substrate mediated channel may be relevant though. Absorption by grain

may be important. Silicate grain mimics in parallel to meteorite derived material studies

Move on to looking at mixtures More realistic representation of interstellar environment

PAHs So far only benzene has been studied, PAHs have greater stability

Photochemistry Only followed Benzene (78 amu) and water (18 amu) mass numbers with

QMS Utilise RAIRS for infrared studies – any evidence for reaction products

Acknowledgements Academic Team

Prof. Martin McCoustra (Heriot-Watt) Dr Wendy Brown (UCL) Dr Helen Fraser (Strathclyde) Prof. Nigel Mason (OU)

Postdocs Dr Mark Collings (Heriot-Watt) Dr Daren Burke (UCL) Dr Anita Dawes, Dr Phil Holtom, Dr Paul Kendall and others (OU)

Students Farah Islam (UCL) Sharon Baillie (Strathclyde Summer Student) Jenny Noble (Strathclyde Summer Student) Any others I’ve missed

Laser for Science Facility Dr Ian Clark Dr Sue Tavender Ruth Webster David

Workshops at RAL and Nottingham