## 1 Introduction  
Time-resolved photoelectron spectroscopy (TRPES) has become a powerful tool for studying the photo-induced reaction dynamics of polyatomic molecules [1]. In TRPES, a pump pulse excites a molecule, triggering a photochemical/photophysical process, and a delayed probe pulse interrogates the dynamics of that process through photoionization. To follow the ultrafast dynamics to the final end product, a high probe photon energy exceeding the electron binding energies (eBEs) of these products is required. Most organic molecules have an eBE of about 10 eV, so detection of internal conversion to the ground electronic state requires vacuum-UV (VUV) pulses. High-harmonic generation (HHG) using an intense 800-nm Ti:sapphire driving laser is a well-established technique to generate VUV pulses [2] and has been employed for photoionization [3]. However, the HHG spectrum consists of numerous harmonic orders that are separated by only 3.1 eV, which causes overlap of photoelectron spectra originating from photoionization by different harmonic orders. To solve this problem, a time-compensated monochromator [4] is typically used to isolate a single-order harmonic from the comb-like HHG spectrum. As an alternative, we have proposed a simple scheme to obtain a high-power single-order harmonic at 90 nm (the 9 th harmonic of a Ti:sapphire laser, _htv_ = 14 eV) using a UV driving laser at 270 nm (the 3 rd harmonic) [5]. A short-wavelength driving laser yields a much higher conversion efficiency in the VUV region [6], and a single-order harmonic can be isolated from other (i.e., higher-order) harmonics easily, because of the greater free spectral range (>9 eV) of the resulting harmonic comb [5].  
Figure 1 shows a schematic energy diagram of TRPES, which measures photoelectron kinetic energy (PKE) as a function of the pump-probe delay time. Here, eBE is the energy required to remove an electron from a molecule, and is given by the difference between the probe photon energy and the measured PKE. The wave packet prepared in the Franck-Condon (FC) region of the excited state starts moving in the direction of descending potential energy (Arrow A in Figure 1), which generally increases the eBE (see Figure 1) [7; 8; 9]. (Rigorously speaking, eBE is given by the difference between the lower (i.e., neutral) and upper (i.e., cationic) state potential energies, and it varies with the transient molecular structure in a complex manner.) When the wave packet reaches the conical intersection (CI)  
region, it transitions to the ground state of the product (Arrow B in Figure 1) or the reactant (Arrow B'), and eBE further increases (typically to around 10 eV) in either case. In the past two decades, probe pulses in the deep-UV (DUV, \(Iw\lesssim 6\) eV) have been commonly employed for TRPES. Though DUV TRPES studies have succeeded in elucidating a variety of photo-induced dynamics [10], CIs and products with eBE higher than 6 eV have been outside of the observation window when using DUV probe pulses (Figure 1).  
We have developed VUV TRPES combined with UV-driven harmonic generation, and have reported a number of TRPES results recently [11; 12; 13; 14; 15; 16; 17]. Our paper is organized as follows: In Section 2, we describe the experimental details of the UV driving laser, the generation and isolation of the 9 th harmonic, and the TRPES apparatus. Next, we present a few representative results taken from our recent TRPES studies in Section 3. Finally, we draw conclusions in Section 4.