For the molecular beam source, a pulsed solenoid valve with an opening diameter of 1 mm (Parker 009-1643-900, driver Iota One 060-0001-900) placed in the source chamber was used. In addition, we have a SiC tube (20 mm length, 1 mm inner diameter, 3 mm outer diameter, WireTrex Limited, Germany) attached to the nozzle orifice. This SiC tube when painted with a thin layer of carbon paste can be resistively heated up to 900 K with 8 watts of electrical power (SI-1). The amount of CO 2 incident on the target surface was estimated for every measurement from the partial pressure changes monitored using the QMS. The QMS was calibrated using an independently calibrated ionization gauge as a reference. Detailed calibration procedures are described in our previous work. The incident beam diameter ranged from 2.2 mm to 2.6 mm (depending on the gas mixture and the nozzle temperature, T n ) and was estimated from the spatial distribution of the O-atom coverage measured after the CO 2 beam exposure for each experiment. The spatial distribution of the O-atom coverage was measured using a combination of surface current and AES (SI-2). This information was used to estimate the incident CO 2 flux on the target surface, which ranged from 0.05 -0.8 ML/sec for different gas mixtures and nozzle conditions. Here 1 ML corresponds to 1.08 × 10 15 atoms cm -2 , the surface density of Cu atoms on a flat Cu(110) surface.