martin mccoustra

School of Chemistry, University of Nottingham,UK 1

Data, Data Everywhere!Data, Data Everywhere!But Which of it to Use? But Which of it to Use?

The Need for Data Validation and Accountability in The Need for Data Validation and Accountability in Laboratory AstrochemistryLaboratory Astrochemistry

Martin McCoustra


A Difficult Topic– Apologies

What is Needed in Data Terms? – Spectroscopic Data

– Kinetic Data

Drawing an Analogy– Atmospheric Chemistry

Where Do We Go From Here?

Outline


A Difficult Topic For Discussion?

Astrochemistry

Spectroscopyand Theory

ChemicalIntuition

ObservationalAstronomy

Reaction RateMeasurements


A Difficult Topic For Discussion?

Laboratory data are crucial for Astrochemistry– Previous presentations at this Workshop

– White Paper from NASA’s 2002 Laboratory Astrophysics Workshop

– Review and foresight articles in the literature• for example

J. M. Greenberg, Surf. Sci., 2002, 500, 793 D. A. Williams and E. Herbst, Surf. Sci., 2002, 500, 823 T. E. Madey, R. E. Johnson and T. M. Orlando, Surf. Sci., 2001,

500, 838.

But Can We Trust the Quality of the Data?


What Types Of Data Are Needed?

Astrochemical Data

Spectroscopic Data– Identification

– Wavelengths ranging from the X-ray to radio

– Measurements made with increasingly higher resolution

– Wide range of chemical species in both the gas and solid phases

Reaction Rate Data– Mechanistic data

supports identification

– Rate data are essential for modelling chemical change

– Not the normal reaction conditions



Spectroscopic Data

Laboratory Measurements– Need to work to

resolutions to match or better observations

– Need to be aware of the impact of data processing on the data

– Look towards a common format for data exchange e.g. JCAMP

Theoretical Calculations– Very effective for isolated

(gas phase) species

– Solids are challenging• Crystalline

DFT (e.g. CASTEP) and periodic HF calculations (e.g. Crystal98)

• Amorphous Cluster Calculations?

MM-QM Combinations?



Solid Phase CO as an illustrative example

LO Mode2143 cm-1

TO Mode2139 cm-1

– Grazing-incidence RAIR spectroscopy from a metal substrate is sensitive to only one plane of polarisation of the IR light

– Increasing resolution clearly shows a splitting of the solid CO stretching vibration



– Observation of splitting may be indicative of a polarised IR source

– Recent work from Pontoppidan et al. (Astron. Astrophys., submitted) suggests this might be the case in some objects

LO TO Solid Phase CO as an illustrative example



Reaction Rate Data

Laboratory Measurements– Reaction rates and their

temperature dependence from 10 K to over 1000 K

– Photon and charged particle interaction cross-sections

– Environment where dynamics is important

– Gases, solids and surfaces

Theoretical Calculations– An excellent tool for

investigating dynamical effects

– Less useful for evaluating thermal reaction rates especially on surfaces and in the solid phase



Temperature dependence of rate of the CN + C2H6 reaction as an illustrative example– It is common for high

temperature (>250 K) rate data to fit the Arrhenius expression

k /

10-1

1 cm

3 m

olec

-1 s

ec-1

3

4

5

6

7

103/(T/K)

4.03.32.01.0

1000 500 300

T/K

Courtesy of Dr. Ian SIms

– But extrapolation to lower temperature can be unreliable!!!!







103 / (T/K)

10.05.03.31.0

k /

cm3

mol

ec-1

sec

-1

10-12

10-11

10-101000 200300

T/K100







103/ (T/K)

50.020.010.01.0

k /

cm3

mol

ec-1

sec

-110-18

10-17

10-16

10-15

10-14

10-13

10-12

10-11

10-10

10-91000 50100

T/K

20



Thermal desorption of water ice as an illustrative example– Measurements of both

activation energy and pre-exponential factor for desorption are required

– Care must be taken with reaction orders on surfaces

– Fraser et al., Mon. Not. Roy. Astron. Soc., 2001, 327, 1165



Thermal desorption of water ice as an illustrative example

RT/E0des

desev

RT/E1des

desev

– Solid films exhibit zero order desorption kinetics

– But monolayers or less may exhibit first or second order desorption

– This impacts on when the film material will reappear in the gas phase


Where Do We Find This Data?

Primary Literature– For the most part, chemical data comes from the extensive

(!!!!) chemical literature but you may have to think “out of the box” to find it

Database Compilations– NIST Databases, Leiden Solid State Spectral Database,

UMIST Kinetics Database

– Issues of database accuracy and maintenance

– “Proprietary” information

Chinese Whispers

Can Anything Make Life Simpler?


Drawing An Analogy

Astrochemistry– Chemical processes

coupled with physical processes

– Chemical reactions in an environment where there is energy, mass and momentum transport

– Driven by radiation from stars

Atmospheric Chemistry– Chemical processes

coupled with physical processes

– Chemical reactions in an environment where there is energy, mass and momentum transport

– Driven by radiation from a star (solar radiation)

In the Broadest Sense, Much More Alike Than Different!


Drawing An Analogy

Spectroscopic Data– Many of the same sources as for astrochemistry but

additional national atmospheric chemistry databases

Reaction Rate Data– A solution to the problem of hunting for data and being

assured of its quality

How Does Atmospheric Chemistry Solve its Data Needs?

http://www.iupac-kinetic.ch.cam.ac.uk/


Drawing An Analogy

An international committee established as part of the IUPAC Division on Physical Chemistry Commission on Chemical Kinetics– Professor R. Atkinson (Riverside, California, USA) Professor D. L.

Baulch (Leeds, UK), Dr. R. A. Cox (Cambridge, UK), Dr. John Crowley (MPI for Chemistry, Mainz, Germany), Dr. R. F. Hampson (NIST, USA), Dr. M. E. Jenkin (Imperial College, UK), Professor J. A. Kerr (Birmingham, UK), Dr. M. J. Rossi (EPFL, Lausanne, Switzerland), Professor J. Troe (Göttingen, Germany)


Drawing An Analogy

An international committee established as part of the IUPAC Division on Physical Chemistry Commission on Chemical Kinetics– Chemical expertise and intuition applied to the collation of

chemical rate data for use in atmospheric chemistry

– Publishes compilations of evaluated data on a regular basis in the Journal of Physical and Chemical Reference Data and on the web

– “Certainty” in the data means that atmospheric modelling can focus more on the physics of the problem


Drawing An Analogy

The reaction HO+H2 as an illustrative example

– Review the literature to collate existing data

– Recommend a preferred value to the user community with a justification


Drawing An Analogy

The interaction of H2O with solid surfaces as an illustrative example– Review not just homogeneous but heterogeneous data in

the collation of existing data

– In some cases, recommendation remains a little open but at least all the relevant data is presented



Data evaluation in the IUPAC model reduces uncertainty in the use of data– Users can focus on the use of the data and the physics of

their problems rather than worrying about data quality

– But it isn’t a panacea!!! Remember the example of heterogeneous chemistry!!!

– Use of the UMIST Kinetics Database across the community goes someway towards this

– But there may be issues with evaluation of the content of that database and with its maintenance



Do we need a community-based effort that applies the IUPAC data evaluation model to the maintenance of the UMIST database?

How would we support this?– National Research Councils

• NO… Danger of data becoming “proprietary”?

– Space agency (NASA/ESA/...) funding• Their science needs it, so should they fund it?

– IAU/IUPAC• Clearly the role of IAU Commission No. 14 on Atomic and

Molecular Data but why duplicate the IUPAC effort?



Open the Discussion to the Floor

martin mccoustra

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