Annual CAVIAR Science Meeting14 - 15 Dec 2009

Annual CAVIAR Science Meeting14 - 15 Dec 2009

Update on Leicester Lab Studies

Mark Watkins, Alex Shillings & Stephen Ball

Update on Leicester Lab Studies

Mark Watkins, Alex Shillings & Stephen Ball

IntroductionIntroduction

• Activities since summer 09– Valve Design– Collaboration - laser apparatus loan from Cambridge– Experiments– Ab initio and training (preliminary work)

• Results of Activities– Valve characterisation– Pulsed detection method– Search for water monomer and dimer spectra– Valve testing with O2 and NO2

• Problems– Issues with slit valve– Number density and detection issues

• Activities since summer 09– Valve Design– Collaboration - laser apparatus loan from Cambridge– Experiments– Ab initio and training (preliminary work)

• Results of Activities– Valve characterisation– Pulsed detection method– Search for water monomer and dimer spectra– Valve testing with O2 and NO2

• Problems– Issues with slit valve– Number density and detection issues

RationaleRationale

Monomer Red = HITRAN 04 (0.24nm FWHM)Dimer (20 cm-1 FWHM), [dimer] = 1% of [H2O]Blue = Schofield & Kjaergaard 2003Green = Schofield et al 2007

Predicted (H2O)2 overtones

= 3 at 960 nm = 4 at 755 nm = 5 at 622 nm

Why? Easiest to detect experimentally (red-shifted from monomer); Atmospheric importance

How? Broadband cavity-based spectroscopy => v long absorption path lengths(Strong H2O monomer bands in field work spectra at 650nm, 570nm, 445nm)

Pulsed nozzle methods => non-equilibrium dimer concentration=> rotation & vibration cooling

Monomer Red = HITRAN 04 (0.24nm FWHM)Dimer (20 cm-1 FWHM), [dimer] = 1% of [H2O]Blue = Schofield & Kjaergaard 2003Green = Schofield et al 2007

Predicted (H2O)2 overtones

= 3 at 960 nm = 4 at 755 nm = 5 at 622 nm

Why? Easiest to detect experimentally (red-shifted from monomer); Atmospheric importance

How? Broadband cavity-based spectroscopy => v long absorption path lengths(Strong H2O monomer bands in field work spectra at 650nm, 570nm, 445nm)

Pulsed nozzle methods => non-equilibrium dimer concentration=> rotation & vibration cooling

What we were doing 6 months ago: LED-BBCEAS around 620 nm ( = 5)

Beautiful spectra of H2O (1% in air), min absorption coeff < 109 cm1

But no H2O (monomer) absorption detectable through pulsed valve.

Issue Solution1. = 5 is weak Move to stronger = 4 overtone 750nm2. Point nozzle Slit nozzle3. [H2O] in source gas Heated source & nozzle

4. CW detection with LED Pulsed detection with broadband laser

What we were doing 6 months ago: LED-BBCEAS around 620 nm ( = 5)

Beautiful spectra of H2O (1% in air), min absorption coeff < 109 cm1

But no H2O (monomer) absorption detectable through pulsed valve.

Issue Solution1. = 5 is weak Move to stronger = 4 overtone 750nm2. Point nozzle Slit nozzle3. [H2O] in source gas Heated source & nozzle

4. CW detection with LED Pulsed detection with broadband laser

Experiment ConceptExperiment Concept

Clocked CCD camera & spectrograph

Pulsed Valve ConceptsPulsed Valve Concepts

Why use a General Valve?• Continuous flow valves generally difficult to deal with and can

easily overload turbo and diffusion pumps• Use a pulsed valve to significantly reduce load on pumps and

reduce resource usage

General valve is an excellent source for a free jet expansion into vacuum

• Can use backing pressures behind a the valve in excess of 10 bar• Can optimise operation of the valve for a given backing pressure• The larger the backing pressure and better optimised the valve,

the better the collisional and expansion cooling when the valve opens to vacuum

• Better cooling conditions allow for non-equilibrium generation of cluster species

• Backing pressure and valve operation can be optimised to bias towards a particular cluster size; higher backing pressure and better optimisation favour larger cluster formation

Why use a General Valve?• Continuous flow valves generally difficult to deal with and can

easily overload turbo and diffusion pumps• Use a pulsed valve to significantly reduce load on pumps and

reduce resource usage

General valve is an excellent source for a free jet expansion into vacuum

• Can use backing pressures behind a the valve in excess of 10 bar• Can optimise operation of the valve for a given backing pressure• The larger the backing pressure and better optimised the valve,

the better the collisional and expansion cooling when the valve opens to vacuum

• Better cooling conditions allow for non-equilibrium generation of cluster species

• Backing pressure and valve operation can be optimised to bias towards a particular cluster size; higher backing pressure and better optimisation favour larger cluster formation

Valve ConceptsValve Concepts

• However, there are certain design limitations when using a valve– The number density in the jet expansion from the valve decreases rapidly within a short

distance of the valve orifice.

– Pulsed operation• Duty cycle is very low compared to a continuous wave (LED) source• Continuous integration of signal leads to very low data acquisition time with respect to dead

time• Upper limit to data collection time per second given by the product of the opening time of the

valve (~ 250 s) and the base repetition rate of the experiment (10 – 20 Hz)

– Low absorption path length• The path length over which absorption takes place is much smaller than the total path length of

the cavity

• However, there are certain design limitations when using a valve– The number density in the jet expansion from the valve decreases rapidly within a short

distance of the valve orifice.

– Pulsed operation• Duty cycle is very low compared to a continuous wave (LED) source• Continuous integration of signal leads to very low data acquisition time with respect to dead

time• Upper limit to data collection time per second given by the product of the opening time of the

valve (~ 250 s) and the base repetition rate of the experiment (10 – 20 Hz)

– Low absorption path length• The path length over which absorption takes place is much smaller than the total path length of

the cavity

Pulsed Broadband LaserPulsed Broadband Laser

• Solution to pulsed valve operation:– Use intense [laser] source instead of LED;

order of magnitude more intense and can be operated in a pulsed manner; use Cambridge broadband emission dye laser

– Data collection can be taken selectively with pulsed operation; use the Cambridge BBCRDS system

– Use of pulsed laser and data acquisition system mean that data collection process is synchronised to the operation of the General Valve

– All dead time eliminated by switching to pulsed operation

• Solution to pulsed valve operation:– Use intense [laser] source instead of LED;

order of magnitude more intense and can be operated in a pulsed manner; use Cambridge broadband emission dye laser

– Data collection can be taken selectively with pulsed operation; use the Cambridge BBCRDS system

– Use of pulsed laser and data acquisition system mean that data collection process is synchronised to the operation of the General Valve

– All dead time eliminated by switching to pulsed operation

Pulsed laser BBCRDS

Pulsed nozzle

Pulsed nozzle

LED-BBCEAS

Nozzle: rep rate 10 - 20 Hz, 180 – 300 s opening time

Pulsed Broadband LaserPulsed Broadband Laser

• Ringdown time of cavity inside vacuum chamber (cavity length = 60 cm) • Ringdown time of cavity inside vacuum chamber (cavity length = 60 cm)

Ring

dow

n tim

e (

s)

Wavelength (nm)

Path = 10 km

Path = 5 km

Jet expansionJet expansion

• Example of a free jet expansion:• Example of a free jet expansion:

Picture taken from: J Chem Phys, 2009, 131, 204312

Barrel Shock

MACH disk region

2 cm

Valve ConceptsValve Concepts

• Number density issues– Number density decrease drastically with

increasing separation from the valve orifice

• Number density issues– Number density decrease drastically with

increasing separation from the valve orifice

3/2

1/12

0

26.3

3/5,12

11

D

XM

Mn

n

effeff

eff

Orifice = 100 micron; 200 micron; 500 micron

Mach number vs distance

Density vs distance

Schematic of a free jet expansionSchematic of a free jet expansion

Most useful spectroscopic region is in the ‘zone of silence’ just prior to the MACH disk in the expansion; vibrationally and rotationally coolest region of the expansion, so will have the highest density of cluster species

• Physical issues with free jet expansion:– We MUST do our detection in the first 1 – 2 mm after the nozzle, after which the number density is

predicted to fall off

Most useful spectroscopic region is in the ‘zone of silence’ just prior to the MACH disk in the expansion; vibrationally and rotationally coolest region of the expansion, so will have the highest density of cluster species

• Physical issues with free jet expansion:– We MUST do our detection in the first 1 – 2 mm after the nozzle, after which the number density is

predicted to fall off

Valve ConceptsValve Concepts

• Valve design:– Use a slit nozzle in preference to a circular nozzle to increase absorption path length

• Valve design:– Use a slit nozzle in preference to a circular nozzle to increase absorption path length

beampath beampath

Slit width = 12 mmDiameter ~ 350 m

Heated ValveHeated Valve

• Must further modify valve in order to increase dimer production– Use a heated reservoir immediately behind valve– Increases water monomer vapour pressure considerably in region immediately behind valve– Allows for increased dimer production in free jet expansion

• Must further modify valve in order to increase dimer production– Use a heated reservoir immediately behind valve– Increases water monomer vapour pressure considerably in region immediately behind valve– Allows for increased dimer production in free jet expansion

H2O reservoir

General Valve

thermal body heating

collar

thermocouple

Valve progressValve progress

• Heated nozzle and temperature controller characterisation

– Both nozzle reservoir and temperature controller builds have been completed

– Temperature controller has been ‘trained’ in air and in vacuum up to ~60oC

– Achieve temperature control over nozzle and reservoir better than ~0.4oC

• Valve characterisation– Tested both slit and circular orifice valves at a range of (i) backing

pressures, (ii) opening times, (iii) temperatures– Attempted to optimise valves in all the conditions above to gain, at

minimum, a reasonable estimate of viable operating parameters under a range of conditions

– Tested in air for sharpness of gas pulse for comparison to performance in vacuum

– Issues with slit nozzle when working at higher backing pressures and opening times, and at high temperature

• Heated nozzle and temperature controller characterisation

– Both nozzle reservoir and temperature controller builds have been completed

– Temperature controller has been ‘trained’ in air and in vacuum up to ~60oC

– Achieve temperature control over nozzle and reservoir better than ~0.4oC

• Valve characterisation– Tested both slit and circular orifice valves at a range of (i) backing

pressures, (ii) opening times, (iii) temperatures– Attempted to optimise valves in all the conditions above to gain, at

minimum, a reasonable estimate of viable operating parameters under a range of conditions

– Tested in air for sharpness of gas pulse for comparison to performance in vacuum

– Issues with slit nozzle when working at higher backing pressures and opening times, and at high temperature

Slit valveSlit valve

• Issues with pulsed valve operation:– The nozzle armature has a flat plate with a rubberised

disk mounted on it. The rubber disk distorts to form a seal against the slit orifice when the valve is shut.

– This caused two immediate problems:• Higher pressures force the flat plate above the rubber

seal against the nozzle; valve becomes inoperable above about a 2 bar pressure differential

• Higher temperatures (above ~ 45oC) caused the rubber seal to become tacky and stick to the slit nozzle, causing operation problems; typically, large pressure fluctuations were initially observed, eventually resulting in loss of operation as the seal became permanently stuck to the slit nozzle

• Solutions:– Buy or manufacture new faceplate that is less sensitive

to temperature– Operate at low pressure – fortunately, this favours

production of smaller clusters– Switch to circular orifice valve as a stopgap

• Issues with pulsed valve operation:– The nozzle armature has a flat plate with a rubberised

disk mounted on it. The rubber disk distorts to form a seal against the slit orifice when the valve is shut.

– This caused two immediate problems:• Higher pressures force the flat plate above the rubber

seal against the nozzle; valve becomes inoperable above about a 2 bar pressure differential

• Higher temperatures (above ~ 45oC) caused the rubber seal to become tacky and stick to the slit nozzle, causing operation problems; typically, large pressure fluctuations were initially observed, eventually resulting in loss of operation as the seal became permanently stuck to the slit nozzle

• Solutions:– Buy or manufacture new faceplate that is less sensitive

to temperature– Operate at low pressure – fortunately, this favours

production of smaller clusters– Switch to circular orifice valve as a stopgap

Synchronising valve & laserSynchronising valve & laser

• Cambridge Apparatus – searching for absorbing species– Collaboration with Alex Shillings (Cambridge)

• Spatial overlap– 1 to 2 mm immediately below the nozzle orifice (optimum clustering &number density

issues) – Left / right translation to intercept laser beam

• Temporal overlap– Laser pulse & gas pulse sync via delay generator– Temporal profile of the gas pulse (optimum clustering when valve first opens)– Can’t use long gas pulses = overload pumps– Optimum opening time varies with backing pressure

Clear that best chance of observing the water monomer & dimer is to optimise valve operation first with a strong absorber

• Cambridge Apparatus – searching for absorbing species– Collaboration with Alex Shillings (Cambridge)

• Spatial overlap– 1 to 2 mm immediately below the nozzle orifice (optimum clustering &number density

issues) – Left / right translation to intercept laser beam

• Temporal overlap– Laser pulse & gas pulse sync via delay generator– Temporal profile of the gas pulse (optimum clustering when valve first opens)– Can’t use long gas pulses = overload pumps– Optimum opening time varies with backing pressure

Clear that best chance of observing the water monomer & dimer is to optimise valve operation first with a strong absorber

Synchronising valve & laserSynchronising valve & laser

– In conjunction with uvizen spectrograph data, we have used the Cambridge clocked CCD camera to search for absorbances in:

– Column abundance per pass: 1 Torr O2 over full cavity length 1 Atm O2 through slit valve

– That we haven’t reliably seen O2 through valve suggests something still not right with timings and/or spatial overlap.

– Search for strongly absobing species to align valve is still in progress

– In conjunction with uvizen spectrograph data, we have used the Cambridge clocked CCD camera to search for absorbances in:

– Column abundance per pass: 1 Torr O2 over full cavity length 1 Atm O2 through slit valve

– That we haven’t reliably seen O2 through valve suggests something still not right with timings and/or spatial overlap.

– Search for strongly absobing species to align valve is still in progress

Absorber Ambient air Reduced pressure inside chamber

Through pulsed valve into chamber at 10-6 Torr

Water monomer Yes - No

Oxygen molecule Yes Yes, down to Ptot = 5 Torr

(20% in air)

Maybe through slit valve (1 Atm, 100% O2)

NO2 diluted in air Yes - No

Water dimer - - No

SummarySummary

• Optimisation of valve parameters proving very difficult due to small absorption, teardrop profile of pulsed expansion and changing number density through beam.

• Currently most likely sub-optimal spatial and temporal overlaps. • Seed a very strong absorber in region of free jet expansion• Pre-amplify laser beam during its second pass through the dye cell – will raise

photon flux by up to an order of magnitude and enable greater signal averaging. • Give ourselves best chance of seeing a detectable absorption:

– (i) slit valve to increase axial overlap with laser beam– (ii) heated valve to increase [H2O] in source gas

• Problems with slit valve – sticking at higher temperatures and interferring with pulsed operation; will require new parts if available or re-manufacture if not

• Optimisation of valve parameters proving very difficult due to small absorption, teardrop profile of pulsed expansion and changing number density through beam.

• Currently most likely sub-optimal spatial and temporal overlaps. • Seed a very strong absorber in region of free jet expansion• Pre-amplify laser beam during its second pass through the dye cell – will raise

photon flux by up to an order of magnitude and enable greater signal averaging. • Give ourselves best chance of seeing a detectable absorption:

– (i) slit valve to increase axial overlap with laser beam– (ii) heated valve to increase [H2O] in source gas

• Problems with slit valve – sticking at higher temperatures and interferring with pulsed operation; will require new parts if available or re-manufacture if not

Undergraduate BSc project: Sadna DonUndergraduate BSc project: Sadna Don

• Ab initio calculations – water dimer and monomer– Initially started as a BSc project, but ran quickly and successfully enough to be of

possible use in predicting properties of some value to our experiments. Currently raw data is being generated and worked up

• Test model chemistry against experiment and current ab initio– Geometries of dimer– Frequencies (harmonic and anharmonic)– Binding energies

• Interaction energies• Dissociation energies• Zero-point corrections

• Test of model chemistry– Comparison of above to experiment in the literature– Comparison of above to ab initio in the literature

• Predict further properties– Use literature basis set extrapolation techniques– Predict extrapolated frequency red-shifts

• VERY PRELIMINARY WORK – not necessarily of any more use than work already in the literature, but worth following up as raw data is available

• Ab initio calculations – water dimer and monomer– Initially started as a BSc project, but ran quickly and successfully enough to be of

possible use in predicting properties of some value to our experiments. Currently raw data is being generated and worked up

• Test model chemistry against experiment and current ab initio– Geometries of dimer– Frequencies (harmonic and anharmonic)– Binding energies

• Interaction energies• Dissociation energies• Zero-point corrections

• Test of model chemistry– Comparison of above to experiment in the literature– Comparison of above to ab initio in the literature

• Predict further properties– Use literature basis set extrapolation techniques– Predict extrapolated frequency red-shifts

• VERY PRELIMINARY WORK – not necessarily of any more use than work already in the literature, but worth following up as raw data is available

Additional PossibilitiesAdditional Possibilities

• Additional experiment – photoacoustic spectra– Very easy to set up– Time soon available on the IR OPO needed for these experiments– Signal against zero background

• Additional experiment – photoacoustic spectra– Very easy to set up– Time soon available on the IR OPO needed for these experiments– Signal against zero background

Pulsed IR laser (OPO/OPA)

2400 2500 2600 2700 2800 2900 3000 3100

Inte

nsi

ty

Wavelength / nm

Water vapour spectrum (taken in air)

Sig

nal

inte

grat

ion

and

an

alys

is