Current CAVIAR activities at Cambridge

A.J.L. Shillings and R.L. Jones

1University of Cambridge, Department of Chemistry, Lensfield Road, Cambridge, CB1 2EW

Contents

•BBCRDS at Leicester

•CRDS Studies

•Further Inferences from BBCRDS measurements

•Update on IRBBCEAS experiments

BBCRDS at Leicester

•BBCRDS experimental system has been moved from Cambridge to Leicester.

•Move completed successfully and the BBCRDS apparatus has been installed in the spectroscopy labs at Leicester.

•BBCRDS system has been optimised for recording spectra inside the vacuum chamber of the pulsed nozzle.

•AJLS travels to Leicester regularly (1 or 2 days per week) to assist with experiments.

•See Leicester talk for further details.

Monochromatic CRDS experiments

• Within the given time window, it has not been possible to measure water spectra (which would provide a thorough test of water monomer spectral databases) at high resolution at Cambridge.

•The monochromatic dye laser (that was on a short term loan) required to perform these experiments has been returned to Corey Evans at Leicester.

•Investigating the possibility that these experiments can be re-scheduled to take place at Leicester in the new year.

•(Cambridge temperature controlled cell to move to Leicester??)

Further Inferences from BBCRDS measurements

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W avenumber cm 1

Dim

erAbs

Cross

Sectioncm2m

olecule1 HWHM = 20 cm-1

HWHM = 100 cm-1

•In addition to differential structure, if dimer transitions are indeed Lorentzian, BBCRDS at 750 nm should see a baseline signal.

Further Inferences from BBCRDS measurements

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erAbsorbancecm1

•Dimer absorbance assuming conditions of measured spectra.

Further Inferences from BBCRDS measurements

•Differential structure implies a minimum HWHM.

•Baseline signal implies a maximum HWHM (with caveats) (for strong distant bands).

Reminder of the key aims of IR BBCEAS

•Typical mirror high reflectivity bandwidth ~ 200 nm.

•Bandwidth of detector (Grating Dependent), Resolution (Slit Dependent).

•First experiments in this region to make use of excellent bandwidth of instrument to particularly target the weakest continuum away from band centres – high sensitivity and broadband approach – novel set of measurements of high [H2O] ‘continuum’ over a range of T’s.

GratingBandwidth /

nmResolution FWHM /

nm

1 ~300 ~1

2 ~100 ~0.3

3 ~20 ~0.1

Initial IR BBCEAS Experiments

550 – 2500nm water bands

BBCRDS

First Measurements (1250 to 1450 nm)

Also investigated this region (900 to 1000 nm)

IR Light Source Difficulties

•Cooled Red light source still not delivered – despite repeated assurances from manufacturer and supplier.

•Have pursued alternative (less ideal) light sources.

•Ocean Optics HL2000HP Tungsten Halogen – used to record spectra shown in the following.

•LEDs – see later.

BBCEAS Spectra (1)

First spectrum recorded at 1150 nm

– water structure clearly present but

rather noisy.

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Away from peak R – path rather short.

Integration time = 0.1s

Wavelength / nm

(Io/

I) -

1

9090 cm-1 8550 cm-1

BBCEAS Spectra (2)

Spectra around 1250 nm – R higher

(and hence path longer), but light

levels lower so need to integrate for

longer - now drifting baseline becomes

an issue.

1180 1200 1220 1240 1260 1280 1300

0 .005

0 .010

0 .015

0 .020

0 .025

0 .030

Ideal region – between the main water monomer absorption bands

– v sensitive measure of the continuum absorption

Wavelength / nm

(Io/

I) -

1

8475 cm-1 7690 cm-1

Integration time = 30s

Next Steps

•Investigate less reflective mirrors – will give much improved signal levels but also reduce path lengths – (100’s m vs several kms).

•Mirrors (R=99.9 (vs 99.99)) are currently being sourced from Layertec mirror coatings.

•Similar effects have been observed in the 950 nm region (the second region invested so far.

•Investigate increasing photon yield from LEDs

LED Experiments (1)

•If used conventionally, LEDs at these wavelengths don’t produce enough light for BBCEAS experiments.

But…

•If immersed in L.N2, LEDs can be driven MUCH harder before failure.

LED Experiments (2)

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Current / mA

Inte

gra

ted

Em

iss

ion

YH170

CY84

•By cooling LEDs and then driving at ~600 mA (instead of 100 mA) light intensity can be increased by an order of magnitude.

LED Experiments (3)

•Need to re-design LN2 dewar to fully investigate the stability of the LED emissions.

CY84 LED

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Current / mA

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tral

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elen

gth

/ n

m

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HW

HM

/ n

m

•Have to be aware of variable spectral properties of LED light

Pure H2O absorption cell

•Designed and constructed new cell and mirror mounts enabling measurements of pure water vapour to be made.

•Using two lab pumps can achieve base (background) pressure < 1mb.

•Currently only at RT but expect to add variable temperature capability soon.

Summary

•BBCRDS system installed and running at Leicester.

•CRDS experiments have been postponed – there may be a possibility to perform these at Leicester in the new year.

•In addition to the minimum HWHM for water dimer transitions inferred from BBCRDS measurements, it is also possible to infer an upper limit from the magnitude of the observed ‘baseline’ signal.

•IR BBCEAS hindered by delayed delivery of light source

•Alternative light sources together with less reflective mirrors will permit measurements to be made.

•New absorption cell designed that will allow measurements of pure H2O to be made.

Outline