Line mixing and collision induced absorption in the A-band of molecular oxygen: catching oxygen in collisions! Wim J. van der Zande, Maria Kiseleva +, Bas van Lieshout, Marko Kamp, Hans Naus, M. Tonkov *, N.N. Filippov * Institute for Molecules and Materials SRON January * St.-Petersburg State University

Nijmegen Science Faculty NMR pavillion (Kentgens et al.) HFML Science faculty: opening 2007 HFML NMRFEL HFML

Contents: Why study the A-band of Molecular Oxygen? Atmospheric relevance and fundamental questions Light – molecule Interaction From idealized two level systems to absorption in a thermal gas: absorption without collisions:Line shapes absorption in between collisions:Line Mixing (1948 Bloembergen) absorption during collisionsCIA (Color of liquid oxygen) Our approach: cavity ring down spectroscopy Testing and improving LM-theories LM and CIA in O 2 -A

Why: ‘SRON’ problems: n(z) Air mass (clouds) (Brigtness T)

LM and CIA in O 2 -A Effects on Satellite Remote Sensing: (a) 15  m CO 2 : fluctuations in brightness T of 10 K (b) up to 5% deviation (systematic error) determining photon paths in A-band because of incomplete knowledge of lineshapes (2005, Yang et al, JQRST) WHY ATTENTION FOR LINESHAPES BEYOND HITRAN

LM and CIA in O 2 -A The O 2 -A Band (780 nm)

LM and CIA in O 2 -A Q: How long does photo-absorption take in a molecule ? A: It depends.. LM and CIA in O 2 -A Molecular Eigenstate: energy infinitely precise Doppler Shift: apparent photon energy changes Molecular Eigenstate: energy infinitely precise

NCAS Absorption without collisions Step one: solve the eigen-energy problem: energies are infinitely well defined Step two: if photon can go in, it also can go out  a finite lifetime of the upper state. The ‘energy’ gets a ‘width’. Step three: the velocity distribution gives an inhomogeneous broadening (each velocity group is ‘independent’) A Boltzmann distribution without collisions (education)....  Voigt.... ‘time’

LM and CIA in O 2 -A Q: How long does photo-absorption take in a molecule in a gas ? A: It depends on collision rates or not.... LM and CIA in O 2 -A Frequent interruption of the photo-absorption process. + h  ?? A) B) If the photon decide to ‘disappear’ when two molecules are ‘intimate’: What happens then? A reference: collision time 0.2 psec time in between collisions at 1 Bar: 50 ps

The role of collisions: interruption of the ‘coherent’ interaction: HITRAN Absorption in between collisions:

From photon-molecule interaction to collisions in gases.... Absorption in between collisions: An idea of the formalism: Dipole operator Line Shape Eigen- energies Fourier Transform If everything is time independent:

From photon-molecule interaction to collisions in gases.... Absorption in between collisions: An idea of the formalism: If only  i is time dependent and exponentially decaying:

From photon-molecule interaction to collisions in gases.... Absorption in between collisions: An idea of the formalism: If you put ‘collisions’ in the Schrodinger Equation, then molecular properties become time dependent. Thus: ‘X’ the dipole operator, and E i,f is no infinitely defined..... And the misery starts.. Line mixing!

Absorption in between collisions: The formalism results only in redistribution of the absorption strength! The line strengths of HITRAN remain good! The line wings become weaker, absorption strength creeps to the center  Atmospheric consequences even in low resolution spectra

LM and CIA in O 2 -A + h  ?? B) If the photon decide to ‘disappear’ when two molecules are ‘intimate’: What happens then? A reference: collision time 0.2 psec time in between collisions at 1 Bar: 50 ps ‘During’ a collision: (I) The Dipole Moment changes in AMPLITUDE (II) The photon energy does not go into the INTERNAL ENERGY only but also redistributes the kinetic energy: no more peaks (III) The relative importance scales with the square of the density/pressure

Cavity Ring Down Spectroscopy The Hunt for LM and CIA in an Experiment: Very sensitive detection technique: looking in the line wings Signals as function of pressure: see below Nearly independent of pressure Nearly quadratic with pressure: one factor is increase in density, one factor is broadening! Voigt: (Hitran) LM model

Cavity Ring Down Spectroscopy 50 cm pressure cell, motor driven mirror alignment p max =10 Bar Mirror reflectivity: % Decay time: 100  s (30 km) Up to 150 times the total oxygen amount in our atmosphere! The Hunt for LM and CIA in an Experiment: requirements: Very sensitive detection technique Signals as function of pressure. Principle: after a nanosecond light pulse in.... Exponential decaying intensity leaking out determined by mirrors and in-cell absorption

Cavity Ring Down Spectroscopy Pressure  Decay time  Fit: decay= a*p + b*p 2 a: Rayleigh scattering b: CIA + Line Mixing (if measured in the far wing) : fixed Each point is the result of ONE exponential decay

Cavity Ring Down Spectroscopy Pressure  Decay time  Fit: decay= a*p + b*p 2 : b contains LM and CIA : fixed LM-model Observation CIA!

Cavity Ring Down Spectroscopy Pressure  Decay time  Fit: decay= a*p + b*p 2 : b contains LM and CIA : fixed CIA: smooth no peaks Above the R branch In between the P lines An imperfection of ?

Cavity Ring Down Spectroscopy Pressure  Decay time  Fit: decay= a*p + b*p 2 : b contains LM and CIA, assuming LM model works : fixed Comparison with Tran/Hartmann (JGR, 2006) FT high pressure

Conclusions We have observed FAR WING ABSORPTION (1) We detect the combination of LM (line shape details) and CIA (2) We observe that (ABC-model: Tonkov) LM model is reasonable in magnitude, not good in details (3) We are confident that we can improve the Line Shape Determinations (4) CRDS does not have the dynamic range to map the full line-shape (5) As other analyses show, the reduction of the far wing absorption due to LM has a significant impact on satellite retrieval of air mass factors (even in low resolution spectra)