That's an interesting point, Caius. Humbert and the two other Cardinals dispatched by Pope Leo IX arrived in Constantinople in early April, 1054. Leo IX died (according to his entry in The Catholic Encyclopedia) on April 19. Humbert laid the bull of excommunication upon the High Altar of Haghia Sophia on July 16, 1054. I'm not sure exactly how long it would have taken news to travel from Rome to Constantinople in the eleventh century, but I'm confident it would have been less than eight weeks -- my guess would be three to four weeks. So Humbert was almost certainly aware of the Pope's death when he carried out the momentous step of excommunication. I suspect the view that he and his colleagues took of the matter was that they had been invested with full powers to act as they saw fit, and that they were empowered to do so until such time as a new pontiff was elected and issued different instructions.