Atmospheric pressure electrospray had been used previously in our laboratory to generate massive cluster ion beam for charged droplet ionization; a method we called electrospray droplet impact (EDI). Previous ion gun that employed atmospheric pressure electrospray lacks adequate beam current and density for surface and interface analysis. To improve the EDI beam performance, we have proposed a technique for producing a stable electrospray of aqueous solution under vacuum. It is well known that vacuum electrospray of volatile liquids is difficult because of the freezing of the liquids by evaporative cooling under vacuum. Vacuum electrospray of aqueous solution was achieved by irradiating the tip of the electrospray emitter with infrared or near-infrared laser to prevent the freezing under high vacuum (<0.05 Pa). A stable and large beam current was also obtained by optimizing the vacuum electrospray conditions. On the basis of these results, the vacuum electrospray can be expected to be a high-performance massive cluster ion gun.