Time-Frequency Analysis in Attosecond Spectroscopy

Abstract: This thesis deals with ultrafast dynamics of electronic processes in rare gas atoms. The processes we explore include photoemission, where we time the emission of electrons moving away from the atomic core following ionization by a photon; and auto-ionization, where the atom spontaneously releases an electron wave-packet following photo-excitation. These processes occur on an attosecond and femtosecond time scale, respectively. The experimental results presented in this thesis have been obtained with the interferometric technique named RABITT, which involves ionizing a target with a train of attosecond pulses and "probing" the event with a weak IR pulse. The attosecond pulse train is generated by focusing an intense laser pulse of femtosecond duration into a gaseous medium. Through a well-known process called high-order harmonic generation, a broadband spectrum of phase locked frequencies are generated in the medium, resulting in a train of pulses with attosecond duration. The special characteristics of this spectrum allows for a quantum interferometer to be conceived by overlapping the train with a weaker replica of the femtosecond IR pulse. Information of the dynamics of the ionization process is imprinted in the interference fringes obtained by varying the delay between the APT and the IR pulse with attosecond precision. In the six papers that this thesis is based on, we investigate three different rare gas systems: helium, neon and argon. We are able to tell which electron, released out of two different shells of neon, escapes first from their parent atom, with a precision of 10 attoseconds or better. In helium, we are able to follow the creation of a wave-packet created by absorption of an attosecond pulse in the vicinity of a resonance and time its decay. We pave the way towards accessing all available information about a similar wave-packet creation in argon, through an angle detection technique with attosecond precision. We then perform a detailed examination of the interferometric technique used in all six papers (RABITT) and determine its limitations in terms of time resolution. The realization of this thesis work involved generating, from a fundamental frequency corresponding to a photon energy of 1.55 eV, high-order harmonics with photon energies exceeding 100 eV. It also involved developing a more stable optical interferometer, optimizing a 2 meter long time-of-flight electron spectrometer, developing scripts for treating and analyzing the data retrieved with the RABITT technique and creating a new interface for acquiring data from the spectrometers.