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R. de Nalda et al.
front plate of the MCP, its gain is gated, so that a selective detection of ion masses
can be achieved. The two-dimensional (2D) mass-selected ion images on the phosphor screen are recorded with a Peltier-cooled 12-bit CCD camera and stored in a
computer. The velocity, and thus the kinetic energy, of the ions, was calibrated using
methyl images produced in the A-band photodissociation of CH 3 I for a long time
delay between the pump and probe pulses, using the known kinetic energy release
of the CH 3 (ν = 0) + I ∗ ( 2 P 1/2 ) and CH 3 (ν = 0) + I( 2 P 3/2 ) channels [4]. In these
conditions, the kinetic energy resolution of the apparatus is better than 100 meV at
1 eV kinetic energy release.
Raw images are projections of the Newton spheres characteristic of the photodissociation process on the plane of the detector. They can be Abel-inverted [7]
in the case of cylindrical symmetry, which is guaranteed if the polarization axes
of all lasers employed are parallel to the plane of the detector. The method used
for inversion was pBasex [8], where polar coordinates are applied for the inversion.
This way, the noise produced in the mathematical procedure is concentrated in the
middle of the image, allowing a clean analysis of the images in the regions of interest.
Time zero, defined as the position of temporal overlap between the pump and
probe lasers on target, and also their cross-correlation function, are given by the
in situ measurement of either the parent ion transient of N,N -diethyl aniline by
(1 + 1 ) REMPI [4] or through multiphoton ionization of Xe [9].
The energy balance for the photodissociation of CH 3 I is given by
hν − D 0 + E i (CH 3 I) = E i (CH 3 ) + ESO[I(
2 P j/2 )] + E kin (CH 3 ) + E kin (I), (4.1)
where v is the frequency of the photolysis laser, D 0 = 2.41 ± 0.03 eV [10] is the
dissociation energy of the C–I bond, E i (CH 3 I) is the internal energy (rotation and
vibration) of the parent molecule in the molecular beam, E i (CH 3 ) is the internal energy of the CH3 fragment, E SO [I( 2 P j/2 )] is the spin-orbit energy for the iodine atom
in the 2P state (for I, E SO = 0 and for I ∗ , E SO = 0.943 eV) [10], and E kin (CH 3 ) and
E kin (I) are the center-of-mass kinetic energies of the methyl and iodine fragments,
respectively, which are linked by the momentum conservation law that translates
into
m I E kin (I) = m CH 3 E kin (CH 3 ).
(4.2)
The angular distributions for each fragment channel, obtained by radial integration
of the corresponding images, have been fitted to the commonly used expression for
one-photon dissociation and (2 + 1) REMPI detection processes [11–13]:
I (θ) =
σ
4π
1 + β 2 P 2 (cos θ) + β 4 P 4 (cos θ) + β 6 P 6 (cos θ)
(4.3)
where θ is the angle between the photofragment recoil direction and the photolysis
laser polarization direction, σ is the absorption cross section (since the experimental setup has been not calibrated for total intensities, σ is treated as a normalization
fitting parameter), β i are anisotropy parameters which reflect the dissociation dynamics and the photofragment polarization, and P i are the Legendre polynomials of
i th order. If no photofragment polarization is expected, Eq. (4.3) can be truncated in
i = 2, and in that particular case, β 2 coincides with the anisotropy parameter, β.
R. de Nalda et al.
front plate of the MCP, its gain is gated, so that a selective detection of ion masses
can be achieved. The two-dimensional (2D) mass-selected ion images on the phosphor screen are recorded with a Peltier-cooled 12-bit CCD camera and stored in a
computer. The velocity, and thus the kinetic energy, of the ions, was calibrated using
methyl images produced in the A-band photodissociation of CH 3 I for a long time
delay between the pump and probe pulses, using the known kinetic energy release
of the CH 3 (ν = 0) + I ∗ ( 2 P 1/2 ) and CH 3 (ν = 0) + I( 2 P 3/2 ) channels [4]. In these
conditions, the kinetic energy resolution of the apparatus is better than 100 meV at
1 eV kinetic energy release.
Raw images are projections of the Newton spheres characteristic of the photodissociation process on the plane of the detector. They can be Abel-inverted [7]
in the case of cylindrical symmetry, which is guaranteed if the polarization axes
of all lasers employed are parallel to the plane of the detector. The method used
for inversion was pBasex [8], where polar coordinates are applied for the inversion.
This way, the noise produced in the mathematical procedure is concentrated in the
middle of the image, allowing a clean analysis of the images in the regions of interest.
Time zero, defined as the position of temporal overlap between the pump and
probe lasers on target, and also their cross-correlation function, are given by the
in situ measurement of either the parent ion transient of N,N -diethyl aniline by
(1 + 1 ) REMPI [4] or through multiphoton ionization of Xe [9].
The energy balance for the photodissociation of CH 3 I is given by
hν − D 0 + E i (CH 3 I) = E i (CH 3 ) + ESO[I(
2 P j/2 )] + E kin (CH 3 ) + E kin (I), (4.1)
where v is the frequency of the photolysis laser, D 0 = 2.41 ± 0.03 eV [10] is the
dissociation energy of the C–I bond, E i (CH 3 I) is the internal energy (rotation and
vibration) of the parent molecule in the molecular beam, E i (CH 3 ) is the internal energy of the CH3 fragment, E SO [I( 2 P j/2 )] is the spin-orbit energy for the iodine atom
in the 2P state (for I, E SO = 0 and for I ∗ , E SO = 0.943 eV) [10], and E kin (CH 3 ) and
E kin (I) are the center-of-mass kinetic energies of the methyl and iodine fragments,
respectively, which are linked by the momentum conservation law that translates
into
m I E kin (I) = m CH 3 E kin (CH 3 ).
(4.2)
The angular distributions for each fragment channel, obtained by radial integration
of the corresponding images, have been fitted to the commonly used expression for
one-photon dissociation and (2 + 1) REMPI detection processes [11–13]:
I (θ) =
σ
4π
1 + β 2 P 2 (cos θ) + β 4 P 4 (cos θ) + β 6 P 6 (cos θ)
(4.3)
where θ is the angle between the photofragment recoil direction and the photolysis
laser polarization direction, σ is the absorption cross section (since the experimental setup has been not calibrated for total intensities, σ is treated as a normalization
fitting parameter), β i are anisotropy parameters which reflect the dissociation dynamics and the photofragment polarization, and P i are the Legendre polynomials of
i th order. If no photofragment polarization is expected, Eq. (4.3) can be truncated in
i = 2, and in that particular case, β 2 coincides with the anisotropy parameter, β.