A 64-year-old Vancouver man is charged with sexually assaulting a Portland convenience store clerk Monday morning, authorities said.Ronald Steven Pickett allegedly forced the 20-year-old female clerk at Plaid Pantry, 735 S.E. 12th Ave., into a back room at knifepoint around 4:30 a.m., then sexually assaulted her. The alleged victim escaped when a customer came into the store, authorities said. A witness who called 911 followed Pickett on foot and provided location updates. Police eventually caught the suspect at Southeast Eighth Avenue and Taylor Street. He had a knife at the time of his arrest, authorities said.Pickett is being held at the Multnomah County Jail on charges of sodomy in the first degree, sex abuse in the first degree, unlawful sexual penetration in the first degree, kidnapping in the first degree, robbery in the first degree, attempted assault in the first degree and coercion. Pickett’s bond is $1,525,000. He will appear in court on the charges Tuesday.Information was not immediately available about what items Pickett allegedly stole from the store to warrant the robbery charge. He is charged with kidnapping because he allegedly held the woman against her will inside the store’s back room, Portland police officials said.