Laser Spectroscopy GroupLaser Spectroscopy Group

Department of PhysicsNUI - University College

CorkCork, Ireland

Research ActivitiesResearch Activities

• Molecular Absorption Spectroscopy• Luminescence Excitation

Spectroscopy• Third-order Nonlinear Susceptibilities

(Z-scan)• Synthesis of Metal Nanoparticles by

Laser Ablation• Surface Enhanced (Resonance) Raman

Spectroscopy

Molecular Absorption Molecular Absorption SpectroscopySpectroscopy

• Cavity Ring-Down Spectroscopy (!)Dye laser based (vis): Static gas, supersonic jets.

• Cavity Enhanced Absorption Spectroscopy

Diode laser based (near IR): Static gas, flow chamber.

• Incoherent Broad-Band Cavity Enhanced Absorption SpectroscopyXenon Lamp based (UV / vis / near IR): Static gas.

[S.E. Fiedler, A. Hese, A.A. Ruth; Chem. Phys. Lett. 371 (2003) 284-294.]

Cavity Ring-Down Cavity Ring-Down SpectroscopySpectroscopy

Cavity-Ring Down (CRD): Highly sensitive direct absorption method for species in the gas-phase.

In CRD spectroscopy the rate rather than the absolute magnitude of a change of intensity is determined.Advantages: • intensity independent (in principle) • very long path-lengths

• high spectral resolution possible • applicable over a wide spectral

range

I0I1I2

mirror (R>0.999)

laser pulse of a dye laser

….. In1

no absorption

crd-1 /

s-1

/ nm

0 20 40 60 80 100

0,0

0,2

0,4

0,6

0,8

1,0

In

I2

I1

I0

rel.

inte

nsi

ty

time / s

1

c)1(

c)(1 R crd

fit I = A exp(-t/crd)

Principle of Cavity Ring-Down Principle of Cavity Ring-Down (CRD)(CRD)

c)1(

c)(1 R crd

fit I = A exp(-t/crd)

Principle of Cavity Ring-Down Principle of Cavity Ring-Down (CRD)(CRD)

I0I1I2

mirror (R>0.999)

laser pulse of a dye laser

….. In2

absorption

crd-1 /

s-1

/ nm

2

0 20 40 60 80 100

0,0

0,2

0,4

0,6

0,8

1,0 I2

I1

I0

rel.

inte

nsi

ty

time / s

Examples of investigations using Examples of investigations using Cavity Ring-Down SpectroscopyCavity Ring-Down Spectroscopy

• Spin forbidden transitions in aromatic thiocarbonyl compounds (in jet and static cell)

A.A. Ruth, W.G. Doherty, R.P. Brint; Chem. Phys. Lett. 352 (2002) 191-201. A.A. Ruth, T. Fernholz, R.P. Brint, M.W.D. Mansfield; J. Mol. Spectr. 214 (2002) 80-86.

• Fast decay dynamics in jet-cooled azulene

A.A. Ruth, E.-K. Kim, A. Hese; Phys. Chem. Chem. Phys. 22 (1999) 5121-5129.

• Nonlinear dynamics of UV multiphoton photolysis products of gaseous naphthalene

A.A. Ruth, E.W. Gash, M. Staak, S.E. Fiedler; Phys. Chem. Chem. Phys. 4 (2002) 5217-5220.

O

S

O

S

Experimental SetupExperimental Setup

Excimer Laser(XeCl, 308 nm)

Dye Laser

PMT

Pump

Probe

Computer

Filter

VacuumCell

HR Mirror

DigitalOscilloscope

GPIB

Lens

Iris

Shutter

CRD MirrorCRD Mirror

Dye LaserDye Laser

Excimer LaserExcimer Laser

CRD MirrorCRD Mirror

Shutter / LensShutter / Lens

PMTPMT

OscillationsOscillations

793 s

494 s483 s

561 sConditions:

Pnap = 0.10 mbarPHe = 77.5 mbar EPuls = 21.4 mJT = 24.1 oC

20 UV Pulses time / hrs.0 1 2 3 4 5

0

1

2

3

4

5

6

7

810

-7

/ cm

-1

Investigations using Cavity Investigations using Cavity Enhanced Absorption Enhanced Absorption

SpectroscopySpectroscopy

High spectral resolution (~60 MHz)

• Water vapour overtones in ambient air (vis)• Formaldehyde (~1.5 m) at 4 mbar

Investigations using Cavity Investigations using Cavity Enhanced Absorption Enhanced Absorption

SpectroscopySpectroscopy

5

10

15

20

~

rel.

abso

rban

ce

/ cm-1

6616.5 6617.0 6617.5 6618.0 6618.5

1511.4 nm 1510.8 nm

rotationally resolved

Z-scans of Pt-octaethylporphyrin Z-scans of Pt-octaethylporphyrin in toluenein toluene

Pt-OEP in toluene (8.6 x 10-5 M) Energy per pulse 8 J.

Closed aperture:’= 7.7 x 10-16 m2 W-1

0.0 0.5 1.0 1.5 2.0 2.5 3.00.8

0.9

1.0

1.1

1.2

1.3

norm

. tra

nsm

issi

on

z / mm

Open aperture:| | = (3.97±0.09) x 10-9 m W-1

0 10 20 30 40 50

1.0

1.1

1.2

1.3

1.4

norm

. tra

nsm

issi

on

z / mm

Synthesis of metal Synthesis of metal nanoparticles by laser nanoparticles by laser

ablationablation

lens

Nd:YAG532 nm

irises x

metal (target)

zy···· ·

······· ···················

·······

··

·

······ ······

cell

H2O

200 400 600 8000.00.20.40.60.81.01.21.4

optic

al d

ensi

ty

/ nm<d>~30 nm

Group MembersGroup Members Academic StaffProf. M. Mansfield (general supervision)

Post-Doctoral FellowsTo be appointed( CEAS and IBBCEAS – multi-component trace gas detection)

PhD StudentsE. Gash (CRD, nonlinear dynamics of naphthalene)M. Staak (CEA, formaldehyde, photolysis products - HO2)K. Lynch (Surface enhanced Raman scattering) One position vacant

Masters StudentsS. O’Brien (z-scan measurements of porphyrin solutions)A. Walsh (z-scan measurements of porphyrin thin films)[ R. Healy (Nanoparticle formation by laser ablation – alloys) ]

Research CollaborationsResearch Collaborations University College Cork – Physics DepartmentProf. D. Nikogosyan, Dr. A. Dragomir(nonlinear absorption measurements)

University College Cork – Chemistry DepartmentProf. P. Brint (LIF, LIP, forbidden transitions, supersonic jets)Prof. J. Sodeau, Dr. J. Wenger (atmospheric chemistry)Dr. J. Holmes, Dr. M. Morris, Prof. T. Spalding(nanoparticles and nanostructures, TEM)Prof. D. Burke (laser assisted electrocatalysis)

Technical University Berlin, GermanyProf. A. Hese, S. Fiedler (CRD spectroscopy, IBBCEAS)

Max-Planck-Inst. Biophys. Chemistry, Göttingen, GermanyProf. J. Troe (liquid phase spectroscopy and kinetics)

Univ. Paris-Sud (Dr. Orphal), Niels-Bohr-Inst. Copenhagen (Prof. Heimburg), Warsaw University (Dr. Borowicz) …. etc.

Future activitiesFuture activities

Multi-component gas analysis using Incoherent Broad-Band Cavity Enhanced Absorption Spectroscopy

Detection of NO2, NO3, O3 , H2O, HONO … (vis)

Alloy nanoparticle synthesisNew materials for SERS

Surface Enhanced (Resonance) Raman ScatteringPorphyrins and proteins on particle surfaces