Updated August 14, 2012
Production of clusters is accomplished by adiabatic expansion of a gas sample, either neat or seeded in a carrier gas, through a pulsed valve. For the formation of mixed clusters, and to initiate reactions of neutral clusters with reactive gases, a pickup source is employed at the exit of the valve. Subsequently, the neutral cluster beam is skimmed into a differentially pumped ionization chamber in which it is crossed by a pulsed laser beam at 90°. The electric field in the ion lens is defined by three parallel plates. Central bores in the plates are covered with a fine Ni mesh to ensure a homogeneous field. The laser beam passes perpendicular to the homogeneous electric field between the first two plates. The potential at the laser focus is typically 4-5 kV with respect to ground. Ions formed by multiphoton absorption are accelerated in an electrostatic field from a positive potential to ground, traverse the drift tube and are detected by a microchannel plate located at a small reflected angle after the reflectron.
If the potential at the second plate is adjusted appropriately, this double-field ion source achieves a first-order space focus. In cases where the reflectron is used to assist in the identification of molecular fragments, we deliberately use the reflectron in a mode which avoids a time-focusing for ions of different kinetic energies.
Output from the Ti:Sapphire femtosecond laser system is used to monitor the dynamics and reactions of the chemical systems of interest. This system consists of a mode-locked Ti:Sapphire laser (Spectra Physics, Tsunami 3955), which is pumped by an Ar-ion laser (Spectra Physics, Beam Lock 2060). The output of this oscillator is amplified in a regenerative Ti:Sapphire amplifier equipped with a pulse stretcher and compressor for chirped pulse amplification, which is pumped by a Nd:YAG laser (Quanta Ray, GCR 150-10). The resulting laser light has a wavelength centered around 800nm (tunable between 720nm and 850nm) at a repetition rate of 10Hz, a pulse duration of about 75 fs and a pulse energy of 3.0 mJ. Generation of the second and third harmonics centered around 400nm and 266nm, greatly enhances the ability to access specific atomic and molecular electronic states. Using this system, the real-time observation of the dynamics and mechanisms of reactions discussed earlier can be ascertained.
Two color pump-probe experiments to study electrolyte dissolution and reaction mechanisms.
To study the dynamics and dissolution mechanisms of atmospherically relevant clusters, we use the pump-probe technique, which relies on the generation of two laser pulses. With our current experimental setup, the fundamental output of the laser system and the second and third harmonics of the fundamental wavelength are available to use as the pump and probe beams. In these experiments the wavelength of the pump laser is tuned into the absorption band of a molecular electronic state, and a probe laser interacting with the cluster at a slightly later time is used to ionize this molecule. Upon dissolution of the probed molecule, an ion-pair is formed and the molecular state is therefore replaced by electronic states of the dissolved cation and anion.
The real time observation of fast ion molecule reactions are performed using a defined laser pulse scheme consisting of an ionization and an excitation pulse. To initiate the reaction, a femtosecond laser pulse in the visible or near UV regime (400 nm or 266 nm) is employed to generate the ions via a multiphoton ionization process. To follow the reaction of these ions with their environment, a harmonic wavelength of the tunable femtosecond laser is used to ionize an intermediate or product species.
Coulomb explosion phenomenon as a detection method for intermediate species.
The technique of using the Coulomb explosion phenomenon to detect intermediate species has been developed by our group. The laser system that has been described here creates a field intensity on the order of 1015 Wcm-2. When molecules are subjected to this intense field, loss of multiple electrons is possible, which gives rise to highly charged ions. When more then one charge center is created in a molecule or cluster, the repulsion of the charges causes the species to fragment. To detect an intermediate species using this technique, the pump beam first excites the species to a higher potential energy surface to start the reaction. The intensity of the probe beam causes the loss of multiple electrons, which starts the Coulomb explosion. Careful analysis of the mass spectra obtained can reveal the nature of the intermediate species.