electrons, are much easier to detect than the change in photon flux through a thin
gas-phase sample. Naturally, the actual decay/action depends on the molecule and
the absorbed energy from a single photon or any given number of photons. The
detectable action may happen on electronically excited states or after the return to
the electronic ground state. The advantage of using the ion-storage ring technique is
that it allows us to track decay processes over a long time span, unlike in a singlepass setup, where distances and ion velocities typically define a fixed and rather
narrow time window.
Action spectroscopy may be used to obtain information about the location of
electronically excited states, as it has indeed been a prime goal for quite some time.
Of particular importance in relation to molecular action is also the fact that
timescales, at which action takes place, branching ratios in decay channels, their
wavelength dependence, etc. bring important information about the photo-initiated
molecular dynamics and enable disclosure of intra-molecular mechanisms, like
internal conversion and non-adiabatic energy-sharing mechanisms between electronic and nuclear degrees of freedom [36].
To facilitate detection of instantaneous action as well as delayed action on the
ms-timescale, different detectors are being applied at the electrostatic ion-storage
ring ELISA. Most recently, a new Secondary Emission Detector (SED) was
installed at ELISA [37]. By this detector, a glass plate is placed into the path of
the laser light and the neutral particles [38]. The glass is transparent for the laser
light used here, but particles create secondary electrons which are accelerated
towards and registered by a channeltron detector. To avoid charging of the glass
and to be able to accelerate the secondary electrons, the plate has a conductive
coating (In 2 O 3 :Sn). The detector arrangement not only increases the time resolution
of photofragmentation studies at ELISA, but most importantly it also allows for a
dead-time free detection of fragments produced immediately after photoabsorption.
A multi-channel plate detector (MCP) is located after the first straight section in
ELISA (see Fig. 5.4) to count neutral dissociation fragments leaving the ring. This
detector only registers fragmentation events that happen later than about a quarter
of a revolution time.
A given experimental setup based on a molecular ‘action’ has a characteristic
time window, where the action may be monitored. In the present experiment, we
detect prompt action with no time delay by the SED arrangement, and at the same
time with the MCP detector we detect action at times down to about a quarter of a
revolution time (here ~15 μs) and up to tens of ms after laser excitation (limited by
the time, where radiative cooling sets in and inhibits the action, i.e., the creation of
neutral particles).
The many milliseconds of storage in vacuum prior to photoexcitation ensure that
we start with a molecule occupying the electronic ground state S 0 . In the case of the
GFP chromophore anion at room temperature, the total vibrational energy is about
0.3 eV (see Fig. 5.5) distributed over the 75 vibrational modes [29], so the average
energy per mode is about 30 cm
À1 , leaving many medium and high-energy
oscillators in the vibrational ground state. At the same time, low-frequency soft
modes may be excited (see Fig. 5.6). Inspection of Fig. 5.5 reveals that a reduction
5 Photo-initiated Dynamics and Spectroscopy of the Deprotonated Green. . .
73
gas-phase sample. Naturally, the actual decay/action depends on the molecule and
the absorbed energy from a single photon or any given number of photons. The
detectable action may happen on electronically excited states or after the return to
the electronic ground state. The advantage of using the ion-storage ring technique is
that it allows us to track decay processes over a long time span, unlike in a singlepass setup, where distances and ion velocities typically define a fixed and rather
narrow time window.
Action spectroscopy may be used to obtain information about the location of
electronically excited states, as it has indeed been a prime goal for quite some time.
Of particular importance in relation to molecular action is also the fact that
timescales, at which action takes place, branching ratios in decay channels, their
wavelength dependence, etc. bring important information about the photo-initiated
molecular dynamics and enable disclosure of intra-molecular mechanisms, like
internal conversion and non-adiabatic energy-sharing mechanisms between electronic and nuclear degrees of freedom [36].
To facilitate detection of instantaneous action as well as delayed action on the
ms-timescale, different detectors are being applied at the electrostatic ion-storage
ring ELISA. Most recently, a new Secondary Emission Detector (SED) was
installed at ELISA [37]. By this detector, a glass plate is placed into the path of
the laser light and the neutral particles [38]. The glass is transparent for the laser
light used here, but particles create secondary electrons which are accelerated
towards and registered by a channeltron detector. To avoid charging of the glass
and to be able to accelerate the secondary electrons, the plate has a conductive
coating (In 2 O 3 :Sn). The detector arrangement not only increases the time resolution
of photofragmentation studies at ELISA, but most importantly it also allows for a
dead-time free detection of fragments produced immediately after photoabsorption.
A multi-channel plate detector (MCP) is located after the first straight section in
ELISA (see Fig. 5.4) to count neutral dissociation fragments leaving the ring. This
detector only registers fragmentation events that happen later than about a quarter
of a revolution time.
A given experimental setup based on a molecular ‘action’ has a characteristic
time window, where the action may be monitored. In the present experiment, we
detect prompt action with no time delay by the SED arrangement, and at the same
time with the MCP detector we detect action at times down to about a quarter of a
revolution time (here ~15 μs) and up to tens of ms after laser excitation (limited by
the time, where radiative cooling sets in and inhibits the action, i.e., the creation of
neutral particles).
The many milliseconds of storage in vacuum prior to photoexcitation ensure that
we start with a molecule occupying the electronic ground state S 0 . In the case of the
GFP chromophore anion at room temperature, the total vibrational energy is about
0.3 eV (see Fig. 5.5) distributed over the 75 vibrational modes [29], so the average
energy per mode is about 30 cm
À1 , leaving many medium and high-energy
oscillators in the vibrational ground state. At the same time, low-frequency soft
modes may be excited (see Fig. 5.6). Inspection of Fig. 5.5 reveals that a reduction
5 Photo-initiated Dynamics and Spectroscopy of the Deprotonated Green. . .
73